Positioning device, capacity controller using positioning device, and speed changing device

ABSTRACT

First and second pistons are provided in a body, the first piston is restricted by the body and the second piston, and the second piston is restricted by the body and a second piston restricting part. Thus, the first piston can be stopped at three positions, and the respective stop positions can be adjusted. Therefore, when rotation speeds of left and right rotating bodies are switched among three levels by a hydraulic motor, a difference of the number of rotations between the left and right rotating bodies can be eliminated, and a vehicle can be suppressed from deviating from a course while it is going straight. Further, a mounting area of the hydraulic pump can be decreased and a structure can be made simple. The rotation speed of the hydraulic motor is changed among the three levels by a simple structure and simple control without using a complex structure such as a servo valve and complex control. Further, the rotation speeds of the rotating bodies are switched among three or more levels by discontinuously positioning the piston not only at three positions but also three or more positions. Furthermore, the mounting area of the hydraulic pump is made small.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a positioning device, and moreparticularly to a positioning device for various types of equipmentincluding control of the capacity of a variable displacement pistonmotor/pump. And, the invention also relates to a speed changing deviceof a rotating body using a hydraulic motor.

Today's machines and equipment have been improved in performance such asspeedup and labor saving on the basis of the employment of a hydraulicpressure. Especially, the piston motor/pump has high performance andefficiency and is important. This piston motor/pump is extensively usedfor construction work machines and many other fields.

The hydraulic motor includes a variable displacement motor whosecapacity can be changed and a fixed displacement motor whose capacitycannot be changed.

The variable displacement motor will be described with reference to FIG.8 to FIG. 10.

FIG. 8 shows a sectional diagram of the hydraulic motor disclosed inJapanese Patent Publication No. 4-42550. This piston motor is changedits capacity between two levels.

Specifically, first seat face 58 a of swash plate 58 housed in casing 55is contacted to inside wall surface 55 a of the casing to position thecapacity at a maximum capacity position. The capacity is positioned at aminimum capacity position when second seat face 58 b of the swash plate58 is contacted to the inside wall surface 55 a of the casing. Thepositioning accuracy of the maximum capacity position and the minimumcapacity position is determined depending on the accuracy of variousparts such as the swash plate 58 and the casing 55.

FIG. 9 shows a sectional diagram of a hydraulic motor different from theone shown in FIG. 8. This motor is an inclined shaft type piston motor.This piston motor changes the capacity position to two levels bypivoting valve plate 46. The valve plate 46 is accommodated in casing52. The capacity position is variable according to the pivoting of thevalve plate 46. Main shaft 51 is an output shaft of the hydraulic motor.The stop position of the valve plate 46 is restricted by minimumcapacity adjusting mechanism 54 and maximum capacity adjusting mechanism53. The minimum capacity adjusting mechanism 54 comprises adjustingscrew 54 b mounted on the casing 52 and nut 54 a for fixing theadjusting screw 54 b to the casing 52. One end of the adjusting screw 54b protrudes from the inside surface of the casing 52, and the other endprotrudes from the outside surface of the casing 52. The maximumcapacity adjusting mechanism 53 is also formed in the same way.

The motor has the minimum capacity when the valve plate 46 comes incontact with the leading end of the adjusting screw 54 b protruded fromthe inside surface of the casing 52. Similarly, the motor has themaximum capacity when the valve plate 46 comes in contact with theleading end of the adjusting screw 53 b protruded from the insidesurface of the casing 52.

The minimum capacity position is adjusted by the minimum capacityadjusting mechanism 54.

Specifically, the fixed state of the adjusting screw 54 b by the nut 54a is released to adjust the screw-in amount of the adjusting screw 54 b,and the adjusting screw 54 b is fixed again to the casing 52 by the nut54 a. Thus, the minimum capacity position is adjusted. The maximumcapacity position is also adjusted in the same way.

FIG. 10 is a sectional diagram of a conventional three-speed motor. InFIG. 10, like reference numerals are used to indicate the likecomponents of FIG. 9, and their descriptions are omitted.

As shown in FIG. 10, first piston 110 and second piston 120 are disposedin body 44. The first piston 110 is connected to the valve plate 46. Infirst pressure-receiving chamber 130, the second piston 120 applies apressure to the first piston 110. Second pressure-receiving chamber 140applies a pressure in a direction to separate the first piston 110 andthe second piston 120 from each other. Third pressure-receiving chamber150 applies a pressure to the first piston 110 in a direction of thesecond piston 120.

The second piston 120 has second piston large-diameter section 120 ahaving a large outside diameter.

Second piston restricting part is configured by the second pistonlarge-diameter section 120 a and body inside wall surface 44 a withwhich the second piston large-diameter section 120 a is contacted.Specifically, the large-diameter section 120 a of the second piston 120moves toward the first piston 110 to contact the large-diameter section120 a with the body inside wall surface 44 a. Thus, the second piston120 is stopped. The first piston 110 comes in contact with the secondpiston 120 whose movement is restricted, and the valve plate 46 ispositioned at a middle position.

Servo valve 60 is a control valve for controlling the supply of pressureoil to the second pressure-receiving chamber 140 and the thirdpressure-receiving chamber 150. The pressure oil discharged from anunillustrated hydraulic pump is supplied to the servo valve 60. And, thepressure oil discharged from an unshown hydraulic pump through anunshown 2-position selector valve is supplied to the firstpressure-receiving chamber 130. Switching of the valve position of the2-position selector valve is controlled so to control the supply andstop of the pressure oil to the first pressure-receiving chamber 130.Thus, the supply of the pressure oil to the first pressure-receivingchamber 130, the second pressure-receiving chamber 140 and the thirdpressure-receiving chamber 150 is controlled so to switch the positionof the first piston 110, namely the position of the valve plate 46,among three positions. Therefore, the capacity position of the motor ischanged among three levels of minimum, middle and maximum capacitypositions. But, the hydraulic motor shown in FIG. 10 is different fromthe one shown in FIG. 9 and does not have an adjusting mechanism foradjusting the minimum and maximum capacity positions.

The aforesaid capacity change of the variable displacement motor is usedto change the speed of HST (hydrostatic transmission) vehicle.

The HST vehicle such as a bulldozer has its left and right runningbodies (wheels or caterpillars) independently driven by left and righthydraulic motors respectively. In other words, the left running body ofthe vehicle is independently driven and changed its speed by a drivemechanism exclusively disposed for the left side Similarly, the rightrunning body of the vehicle is independently driven and changed itsspeed by a drive mechanism exclusively disposed for the right side. Eachdrive mechanism comprises a hydraulic pump and a hydraulic motor.

When the HST vehicle is instructed to run at the same number of left andright rotations, namely to run straight, it causes a deviation from thecourse if the left and right motor capacities are different.

Therefore, the HST vehicle whose left and right running bodies areindependently driven is demanded to have improved accuracy for thecapacity control of the left and right variable displacement pistonmotors so to run without causing a deviation from the course whenstraight-ahead running is instructed.

And, it is also demanded to simplify a device for controlling thecapacity of the variable displacement piston motor.

Besides, it is also said generally that the installing space for thehydraulic equipment mounted to the HST vehicle is limited. Therefore, itis demanded to decrease a mounting area of the hydraulic equipment.

But, there is not any conventional variable displacement piston motorwhich satisfies the aforesaid demands. Descriptions will be made asfollows.

The piston motor of the aforesaid publication shown in FIG. 8 is mainlyapplied to a vehicle such as a hydraulic excavator requiring two speedsof high and low. The hydraulic excavator adjusts the pump capacity onlyand does not adjust the motor capacity when the pump capacity isadjusted in order to prevent the deviation from the course because itsworkability is better than adjusting the motor capacity in view of themounted positions of the pump and the motor.

The HST vehicle often adjusts the motor only, because its workability isvery poor when the pump capacity is adjusted to prevent the deviationfrom the course in view of the mounted positions of the pump and themotor.

When the hydraulic motor shown in FIG. 9 is applied to the HST vehicle,a difference in rotation speeds between the left and right hydraulicmotors is remedied by adjusting the respective capacity positions of theleft and right hydraulic motors. Therefore, a deviation from the courseof the vehicle can be remedied.

But, the adjustment of the capacity position of the hydraulic motorshown in FIG. 9 is limited to two positions of minimum and maximumcapacity positions. When this hydraulic motor is mounted to the HSTvehicle, there is a problem of necessity to enlarge the maximumdischarge capacity of the hydraulic pump for supplying the pressure oilto the hydraulic motor. This point will be described with reference toFIG. 11.

FIG. 11(a) shows the properties of a hydraulic motor (hereinafter calledthe “2-speed motor”) which changes its capacity position between twopositions. In FIG. 11(a), the horizontal axis indicates the vehiclespeed, and the vertical axis indicates traction (torque). The shortdashed line in FIG. 11(a) indicates the property of the hydraulic motorwhen its capacity is adjusted to minimum capacity q Mmin, and the solidline indicates the property of the hydraulic motor when its capacity isadjusted to maximum capacity q Mmax.

Meanwhile, FIG. 11(b) shows the properties of a hydraulic motor(hereinafter called the “3-speed motor”) which changes its capacityposition to three positions. In FIG. 11(b), the horizontal axisindicates the vehicle speed, and the vertical axis indicates traction(torque). The short dashed line in FIG. 11(b) indicates the property ofthe hydraulic motor when its capacity is adjusted to the minimumcapacity q Mmin, the solid line indicates the property of the hydraulicmotor when its capacity is adjusted to the maximum capacity q Mmax, andthe alternate long and short dash line indicates the property of thehydraulic motor when its capacity is adjusted to middle capacity qMmean. In FIGS. 11(a) and 11(b), the region between vehicle speeds V1and V2 is a work region that work is mainly conducted. The regionbetween vehicle speeds V2 and V3 is a running region that the vehiclemainly runs.

In FIGS. 11(a), (b), traction at a low speed (V1) becomes maximum MAX.In designing the vehicle or the motor, the maximum capacity q Mmax ofthe hydraulic motor is determined depending on a level of the maximumtraction MAX.

The 2-speed motor shown in FIG. 11(a) needs that the vehicle speed isadjusted to the maximum vehicle speed V2 of the work region with themaximum capacity q Mmax retained.

Pump capacity Q Pmax of the hydraulic pump is determined by thefollowing equation (2). In the following equation, it is determined thatthe engine speed is NE, the motor rotation speed is NM, the pumpefficiency is ηPV, and the motor efficiency is ηMV.

Q Pmax·NE·ηPV=q Mmax·NM/ηMV  (1)

Q Pmax=(q Mmax·NM/NE)·(1/ηMV·ηPV)  (2)

The 3-speed motor shown in FIG. 11(b) needs the vehicle to have themaximum vehicle speed V2 of the work region with the middle capacity qMmean retained. Pump capacity Q′ Pmax of the hydraulic pump isdetermined by the following equation (4).

Q′Pmax·NE·ηPV=q Mmean·NM/ηMV  (3)

Q′Pmax=(q Mmean·NM/NE)·(1/ηMV·ηPV)  (4)

The above equations (2) and (4) are compared as follows:

q Mmax>q Mmean  (5),

then,

Q Pmax>Q′Pmax  (6)

Therefore, when the 2-speed motor is used, the maximum dischargecapacity of the hydraulic pump must be made larger as compared with thecase of using the 3-speed motor.

Thus, when the 2-speed motor shown in FIG. 9 is mounted to the HSTvehicle, there is a disadvantage that the hydraulic pump is required tohave a larger maximum discharge capacity. In other words, the 2-speedmotor must be designed to have a larger hydraulic pump than the 3-speedmotor has.

When the conventional 3-speed hydraulic motor shown in FIG. 10 ismounted to the HST vehicle which has its left and right running bodiesprovided with the drive mechanisms comprising the hydraulic pump and thehydraulic motor respectively so that the left and right running bodiesare independently driven, the capacity positions of the left and righthydraulic motors cannot be adjusted. Therefore, the vehicle may deviatefrom the course due to a difference in the number of rotations betweenthe left and right hydraulic motors.

The first piston 110 and the second piston 120 shown in FIG. 10 have adifferent outside diameter. And, the same piston has a different outsidediameter depending on its portions. Therefore, the first piston 110 andthe second piston 120 have a complex structure. Besides, the body 44 foraccommodating the pistons 110, 120 has a complex structure.

It is easy to apply the adjusting mechanism of FIG. 9 to the techniqueof FIG. 10. But, the adjustment cannot be made at the middle capacityposition. Therefore, there is still a problem that a deviation from thecourse is caused when the vehicle goes straight at the middle capacityposition.

Therefore, it is a first object of the present invention to provide apositioning device which has a simple structure and can adjust at allthree positions and a motor/pump using this positioning device.

And, it is a second object of the invention to provide a speed changingdevice for left and right rotating bodies using a hydraulic motor whichcan remedy a difference in the number of rotations between the left andright rotating bodies when the rotation speeds of the left and rightrotating bodies are changed among three levels.

As described above, when the 2-speed motor is used in order to obtainthe same maximum vehicle speed V2, it is necessary to increase themaximum discharge amount of the hydraulic pump and to enlarge the sizeof the hydraulic pump as compared with the case of using the 3-speedmotor. This tendency becomes more conspicuous when the number of changesof the hydraulic motor speed is increased to more multiple levels.Therefore, when the number of speed changes of the hydraulic motor isincreased to three levels or more, the maximum discharge amount of thehydraulic pump can be made smaller and the size of the hydraulic pumpcan be decreased as compared with a case that the 2-speed motor is used.In other words, when the positioning number of the positioning device isincreased to three or more, the size of the hydraulic equipment such asthe hydraulic pump can be made smaller as compared with a case that thepositioning device having the positioning number two is used.

But, the structures of the positioning device and hydraulic motor andthe control become complex because the position of the first piston 110is continuously changed by the servo valve 60 according to the prior artof FIG. 10. The prior art of FIG. 10 is a technique to position therotation speed of the hydraulic motor at three positions by thepositioning device. And, there was not a technology that the rotationspeed of the hydraulic motor is positioned at four levels or more by thepositioning device.

It is a third object of the present invention to make positioning atthree positions or more by a simple structure and simple control withoutusing the complex structure such as a servo valve and control and tochange the rotation speed of the hydraulic motor to three levels ormore.

OBJECTS AND SUMMARY OF THE INVENTION

In order to achieve the first object of the invention, a first inventionof the present invention comprises:

a first piston (11) and a second piston (12) in a body (44), the firstpiston (11) having a position restricted by the body (44) and a positioncorresponding to the position of the second piston (12) as stoppositions, and the second piston (12) having a position restricted bythe body (44) and a position restricted by second piston restrictingpart (40 b) as stop positions;

a middle position adjusting mechanism (40) for adjusting the stopposition of the second piston (12) restricted by the second pistonrestricting part (40 b);

a first pressure chamber (13) for applying a pressure to the secondpiston (12) in a direction of the first piston (11);

a second pressure chamber (14) for applying a pressure in a direction toseparate the first piston (11) and the second piston (12) from eachother; and

a third pressure chamber (15) for applying a pressure to the firstpiston (11) in a direction of the second piston (12).

The first invention will be described with reference to FIG. 1, FIG. 2and FIG. 3.

The body 44 is provided with the first piston 11 and the second piston12. The first piston 11 stops at the position restricted by the body 44and the position corresponding to the position of the second piston 12.The second piston 12 stops at the position restricted by the body 44 andthe position restricted by the second piston restricting part 40 b. Thefirst pressure chamber 13 applies a pressure to the second piston 12 ina direction of the first piston 11. The second pressure chamber 14applies a pressure in a direction to separate the first piston 11 andthe second piston 12 from each other. The third pressure chamber 15applies a pressure to the first piston 11 in a direction of the secondpiston 12. And, the supply of the pressure oil to the first, second andthird pressure-receiving chambers 13, 14 and 15 is controlled, so thatthe first piston 11 is positioned at the middle position to come incontact with the second piston 12 whose movement is restricted by thesecond piston restricting part 40 b. This state is shown in FIG. 2.

And, the restriction position of the second piston 12 by the secondpiston restricting part 40 b is adjusted by the middle positionadjusting mechanism 40. Therefore, the stop position of the first piston11 at the middle position can be adjusted.

The second invention comprises:

first piston (11) and second piston (12) in body (44), the first piston(11) and the second piston (12) having the same outside diameter, thefirst piston (11) having a position restricted by the body (44) and aposition corresponding to the position of the second piston (12) as stoppositions, and the second piston (12) having a position restricted bythe body (44) and a position restricted by the second piston restrictingpart (40 b) as stop positions;

a first pressure chamber (13) for applying a pressure to the secondpiston (12) in a direction of the first piston (11);

a second pressure chamber (14) for applying a pressure in a direction toseparate the first piston (11) and the second piston (12) from eachother; and

a third pressure chamber (15) for applying a pressure to the firstpiston (11) in a direction of the second piston (12), wherein:

the first and second pistons (11, 12) are positioned according to adifference in pressure-receiving areas to which the pressures of thefirst, second and third pressure chambers (13, 14, 15) are applied.

The second invention will be described with reference to FIG. 1, FIG. 2and FIG. 3.

The body 44 is provided with the first piston 11 and the second piston12. The first piston 11 stops at the position restricted by the body 44and the position corresponding to the position of the second piston 12.The second stop piston 12 stops at the position restricted by the body44 and the position restricted by the second piston restricting part 40b. The first pressure chamber 13 applies a pressure to the second piston12 in a direction of the first piston 11. The second pressure chamber 14applies a pressure to the first piston 11 in a direction to separate thefirst piston 11 and the second piston 12 from each other. The thirdpressure chamber 15 applies a pressure to the first piston 11 in adirection of the second piston 12. And, the supply of the pressure oilto the first, second and third pressure-receiving chambers 13, 14 and 15is controlled to position the first piston 11 at the maximum positionaway from the second piston 12. This state is shown in FIG. 1. The firstpiston 11 is positioned at the middle position to come in contact withthe second piston 12 whose movement is restricted by the second pistonrestricting part 40 b. This state is shown in FIG. 2. And, the firstpiston 11 is positioned at the minimum position to come in contact withthe second piston 12 whose movement is not restricted by the secondpiston restricting part 40 b. This state is shown in FIG. 3.

The first piston 11 and the second piston 12 are designed to have thesame outside diameter. The first piston 11 is positioned at the maximum,middle or minimum position depending on pressure-receiving areadifferences ((S2−S3), (S1−S3)) among area S1 of pressure-receivingsurface 12 b of the second piston 12 to which the pressure oil of thefirst pressure chamber 13 is applied, area S2 of pressure-receivingsurface 11 a of the first piston 11 to which the pressure oil of thesecond pressure chamber 14 is applied and area S3 of pressure-receivingsurface 11 b of the first piston 11 to which the pressure oil of thethird pressure chamber 15 is applied.

According to the second invention, because the first piston 11 and thesecond piston 12 are designed to have the same outside diameter, thefirst piston 11 and the second piston 12 can be formed to have a simplestructure, and hole 70 for accommodating these pistons 11, 12 can beconfigured to have a simple structure with the same diameter along anyparts of the hole 70. Therefore, there are obtained effects such asfacilitation of a process to produce the positioning device.

A third invention relates to the first and second inventions wherein thefirst piston (11) is connected to a capacity control member of thevariable displacement piston motor/pump to control a capacity positionof the variable displacement piston motor/pump.

The third invention will be described with reference to FIG. 4, FIG. 5and FIG. 6.

According to the third invention, the first piston 11 is connected tovalve plate 46 of the variable displacement piston motor (pump). Whenthe first piston 11 is positioned, the valve plate 46 is positioned atthe corresponding position, and the capacity position of the variabledisplacement piston motor (pump) is positioned.

Accordingly, the hydraulic motor of the third invention can adjust themiddle capacity position. Therefore, when two hydraulic motors are used,a difference of the number of rotations between the left and righthydraulic motors can be eliminated when the hydraulic motors are changedtheir speeds to the middle capacity position. Thus, a deviation from thecourse when running straight can be prevented. Besides, according to thethird invention, the hydraulic pumps can be designed to have a smallcapacity and can be made small in size because the capacity position ischanged among three levels of the minimum capacity position, the maximumcapacity position and the middle capacity position. Accordingly, thehydraulic equipment can be installed in a limited space.

According to the third invention, because the first piston 11 and thesecond piston 12 are designed to have the same outside diameter, theycan be made to have a simple structure, and hole 70 for accommodatingthese pistons 11, 12 can be formed to have a simple structure having thesame diameter at any part of it. Accordingly, there are obtained effectssuch as facilitation of a process to produce the positioning device.

A fourth invention relates to the first and second inventions, whereinthe first piston (11) is connected to a capacity control member of thevariable displacement piston motor/pump to control a capacity of thevariable displacement piston motor/pump, and wherein the capacitycontroller comprises adjusting means (54) for adjusting a minimumcapacity of the variable displacement piston motor/pump.

The fourth invention will be described with reference to FIG. 4, FIG. 5and FIG. 6.

According to the fourth invention, the first piston 11 is connected tovalve plate 46 of the variable displacement piston motor (pump). Afterthe first piston 11 is positioned, the valve plate 46 is positioned atthe corresponding position, and the capacity position of the variabledisplacement piston motor (pump) is positioned.

And, the movement of the first piston 11 which is moved to the minimumcapacity position is restricted by the minimum capacity positionrestricting means 54 b. The restricting position of the first piston 11is adjusted by the adjusting means 54.

Thus, the hydraulic motor of the fourth invention can adjust the middlecapacity position. Besides, according to the fourth invention, thecapacity of the hydraulic pump can be made small and the hydraulic pumpcan be made small accordingly, because the capacity position is changedin three levels of the minimum capacity position, the maximum capacityposition and the middle capacity position. Therefore, the hydraulicequipment can be installed in a limited space.

According to the fourth invention, because the first piston 11 and thesecond piston 12 are designed to have the same outside diameter, theycan be made to have a simple structure, and hole 70 for accommodatingthese pistons 11, 12 can be formed to have a simple structure having thesame diameter at any part of it. Accordingly, there are obtained effectssuch as facilitation of a process to produce the positioning device.

Besides, according to the fourth invention, the position of the firstpiston 11 which is positioned at the minimum capacity position isadjusted. The minimum capacity position is susceptible to the hydraulicmotor (pump). According to the fourth invention, the minimum capacityposition which is largely different and variable among individuals canbe adjusted readily.

To achieve the second object, a fifth invention is a speed changingdevice of rotating bodies using hydraulic motors, comprising left andright variable displacement hydraulic motors (9), (9′) which arerespectively installed for left and right rotating bodies and drive torotate the left and right rotating bodies; hydraulic pumps (3), (3′)which respectively supply pressure oil to the left and right hydraulicmotors (9), (9′); and speed switching means which changes rotationspeeds of the left and right rotating bodies among three levels ofrotation speeds by switching capacity positions of the left and rightvariable displacement hydraulic motors (9), (9′), among three capacitypositions, wherein:

adjusting means is provided for adjusting so that the rotation speeds ofthe left and right rotating bodies are made identical for each of thethree levels of rotation speeds of the left and right rotating bodies.

The fifth invention will be described with reference to FIG. 1 and FIG.13.

When the capacity positions of the left and right variable displacementhydraulic motors 9, 9′ are switched to the minimum capacity position,the rotation speeds of the left and right rotating bodies 100, 100′ areswitched to the maximum rotation speed. At this point, the hydraulicmotors 9, 9′ are adjusted to have the same minimum capacity position bythe adjusting means 54. Thus, the left and right rotating bodies 100,100′ have the same rotation speed.

When the capacity positions of the left and right variable displacementhydraulic motors 9, 9′ are switched to the middle capacity position, therotation speeds of the left and right rotating bodies 100, 100′ areswitched to the middle rotation speed. At this point, the left and righthydraulic motors 9, 9′ are adjusted to have the same middle capacityposition by the adjusting means 40. Thus, the left and right rotatingbodies 100, 100′ have the same rotation speed.

When the capacity positions of the left and right variable displacementhydraulic motors 9, 9′ are switched to the maximum capacity position,the rotation speeds of the left and right rotating bodies 100, 100′ areswitched to the minimum rotation speed. At this point, the left andright hydraulic motors 9, 9′ are adjusted to have the same maximumcapacity position by the adjusting means 53. Thus, the left and rightrotating bodies 100, 100′ have the same rotation speed.

According to the fifth invention, when the rotation speeds of the leftand right rotating bodies 100, 100′ are switched among the three levelsby the hydraulic motors 9, 9′, a difference of the number of rotationsbetween the left and right rotating bodies 100, 100′ can be eliminated.

Besides, according to the fifth invention, the capacity of the hydraulicpump can be made small, and the hydraulic pump can be made small in sizebecause the capacity positions of the left and right variabledisplacement hydraulic motors 9, 9′ are changed among the three levels.Therefore, the cost of hydraulic equipment is reduced, and the hydraulicequipment can be installed in a limited space. And, the vehicleperformance can be improved because the hydraulic pumps and thehydraulic motors can be used under the conditions efficient for thepressures and capacities of the hydraulic pump and the hydraulic motor.

A sixth invention relates to the fifth invention, wherein the speedswitching means switches the rotation speed to

a first rotation speed at which the capacity positions of the left andright variable displacement hydraulic motors (9), (9′) become a maximumcapacity position, and the rotation speeds of the left and rightrotating bodies become a minimum speed;

a second rotation speed at which the capacity positions of the left andright variable displacement hydraulic motors (9), (9′) become a middlecapacity position, and the rotation speeds of the left and rightrotating bodies become a middle speed; and

a third rotation speed at which the capacity positions of the left andright variable displacement hydraulic motors (9), (9′) become a minimumcapacity position, and the rotation speeds of the left and rightrotating bodies become a maximum speed; and

automatically switches between the first rotation speed and the secondrotation speed.

According to the sixth invention, a difference of the number ofrotations between the left and right rotating bodies 100, 100′ can beeliminated when the rotation speeds of the left and right rotatingbodies 100, 100′ are switched among the three levels by using thehydraulic motors 9, 9′ in the same way as the fifth invention.

Besides, according to the sixth invention, the switching between thefirst rotation speed and the second rotation speed is madeautomatically. The range between the first rotation speed and the secondrotation speed is a work area of a low rotation speed with high torque.According to a seventh invention, the manual speed change in the workregion requiring torque is unnecessary, and the operability in the workregion can be improved.

The seventh invention relates to the fifth invention, wherein the speedswitching means switches the rotational speed to:

a first rotation speed at which the capacity positions of the left andright variable displacement hydraulic motors (9), (9′) become a maximumcapacity position, and the rotation speeds of the left and rightrotating bodies become a minimum speed;

a second rotation speed at which the capacity positions of the left andright variable displacement hydraulic motors (9), (9′) become a middlecapacity position, and the rotation speeds of the left and rightrotating bodies become a middle speed; and

a third rotation speed at which the capacity positions of the left andright variable displacement hydraulic motors (9), (9′) become a minimumcapacity position, and the rotation speeds of the left and rightrotating bodies become a maximum speed; and

automatically switches between the second rotation speed and the thirdrotation speed.

According to the seventh invention, a difference of the number ofrotations between the left and right rotating bodies 100, 100′ can beeliminated when the rotation speeds of the left and right rotatingbodies 100, 100′ are switched among the three levels by the hydraulicmotors 9, 9′ in the same way as the fifth invention.

Besides, the second rotation speed and the third rotation speed areswitched automatically according to the seventh invention. The rangebetween the second rotation speed and the third rotation speed is arunning region of the high rotation speed with low torque. According tothe third invention, the manual speed change in the running regionrequiring the rotation speed is unnecessary, and the operability in therunning region can be improved.

An eighth invention relates to the fifth invention, wherein the speedswitching means switches the rotation speed to:

a first rotation speed at which the capacity positions of the left andright variable displacement hydraulic motors (9), (9′) become a maximumcapacity position, and the rotation speeds of the left and rightrotating bodies become a minimum speed;

a second rotation speed at which the capacity positions of the left andright variable displacement hydraulic motors (9), (9′) become a middlecapacity position, and the rotation speeds of the left and rightrotating bodies become a middle speed; and

a third rotation speed at which the capacity positions of the left andright variable displacement hydraulic motors (9), (9′) become a minimumcapacity position, and the rotation speeds of the left and rightrotating bodies become a maximum speed; and

manually switches among the first rotation speed, the second rotationspeed and the third rotation speed.

According to the eighth invention, a difference of the number ofrotations between the left and right rotating bodies 100, 100′ can beeliminated when the rotation speeds of the left and right rotatingbodies 100, 100′ are switched among the three levels by using thehydraulic motors 9, 9′ in the same way as the fifth invention.

Besides, the switching among the first rotation speed, the secondrotation speed and the third rotation speed is made manually accordingto the eighth invention. According to a ninth invention, when it is notdesirable to automatically switch the capacity of the hydraulic motor,the switching can be effected manually as desired.

A ninth invention relates to the fifth invention, wherein the speedswitching means switches the rotational speed to:

a first rotation speed at which the capacity positions of the left andright variable displacement hydraulic motors (9), (9′) become a maximumcapacity position, and the rotation speeds of the left and rightrotating bodies become a minimum speed;

a second rotation speed at which the capacity positions of the left andright variable displacement hydraulic motors (9), (9′) become a middlecapacity position, and the rotation speeds of the left and rightrotating bodies become a middle speed; and

a third rotation speed at which the capacity positions of the left andright variable displacement hydraulic motors (9), (9′) become a minimumcapacity position, and the rotation speeds of the left and rightrotating bodies become the maximum speed; and

selects either automatic switching between the first rotation speed andthe second rotation speed or automatic switching between the secondrotation speed and the third rotation speed.

According to the ninth invention, a difference of the number ofrotations between the left and right rotating bodies 100, 100′ can beeliminated when the rotation speeds of the left and right rotatingbodies 100, 100′ are switched among the three levels by using thehydraulic motors 9, 9′ in the same way as the fifth invention.

Besides, the selection can be made between the automatic switchingbetween the first rotation speed and the second rotation speed and theautomatic switching between the second rotation speed and the thirdrotation speed. The range between the first rotation speed and thesecond rotation speed is a work area of a low rotation speed with hightorque. The range between the second rotation speed and the thirdrotation speed is a running area of a high rotation speed with lowtorque. According to a tenth invention, when the automatic speed changein the work area is selected depending on the use conditions of thehydraulic motor, the operability in the work area can be enhanced, andthe speed change in the running area can be manually made as desired.When the automatic speed change in the running area is selecteddepending on the use conditions of the hydraulic motor, the operabilityin the running area is enhanced, and the speed change in the work areais manually made as desired.

In order to achieve the third object, a tenth invention is a positioningdevice for changing a position of a subject to be positioned dependingon a moved position of a piston (11), which comprises:

the piston (11) which moves between both stroke end positions to changethe position of the subject to be positioned from a minimum position toa maximum position;

one or two or more restricting members (12, 67) which are positioned atone or two or more middle positions between both the stroke endpositions to restrict the movement of the piston (11) at one or two ormore middle positions; and

position control means which changes the position of the subject to bepositioned among three or more positions by the piston (11) and therestricting members (12, 67).

The tenth invention will be described with reference to FIG. 16 and FIG.17(d). The piston 11 moves between the stroke end position shown in FIG.17(a) and the stroke end position shown in FIG. 17(d). The first piston11 moves between both the stroke end positions to change the position ofthe subject to be positioned from the maximum position to the minimumposition.

The third piston 67 is positioned at a first middle position as shown inFIG. 17(b), and the second piston 12 is positioned at a second middleposition as shown in FIG. 17(c). When the first piston 11 comes incontact with the third piston 67, the movement of the first piston 11 isrestricted at the first middle position, and when the first piston 11comes in contact with the second piston 12, the movement of the firstpiston 11 is restricted by the second middle position.

The first piston 11, the second piston 12 and the third piston 67 arediscontinuously positioned at the respective positions.

In other words, when the first piston 11 is positioned at one stroke endposition as shown in 17(a), the position of the subject to be positionedbecomes the maximum position.

When the third piston 67 is positioned at the first middle position andthe first piston 11 is positioned to come in contact with the thirdpiston 67 as shown in FIG. 17(b), the position of the subject to bepositioned becomes the first middle position.

When the second piston 12 is positioned at the second middle positionand the first piston 11 is positioned to come in contact with the secondpiston 12 as shown in FIG. 17(c), the position of the subject to bepositioned becomes the second middle position.

When the first piston 11 is positioned at the other stroke end positionas shown in FIG. 17(d), the position of the subject to be positionedbecomes the minimum position.

According to the tenth invention, the position of the subject to bepositioned can be changed among three or more positions bydiscontinuously positioning the first piston 11, the second piston 12and the third piston 67 at the respective positions. Therefore, thesubject to be positioned can be positioned among three or more levels bya simple structure and simple control without using a complex structuresuch as a servo valve and complex control. The hydraulic equipment suchas the hydraulic pump can be made small in size because the number ofpositioning by the positioning device increases to three or more.

To achieve the third object, an eleventh invention is a speed changingdevice of rotating bodies using a hydraulic motor, comprising a variabledisplacement hydraulic motor (9) which rotatably drives the rotatingbodies; a hydraulic pump (3) which supplies pressure oil to the variabledisplacement hydraulic motor (9); and speed switching means whichchanges rotation speeds of the rotating bodies by changing a capacityposition of the variable displacement hydraulic motor (9), wherein thespeed switching means includes:

a piston (11) which changes the capacity position of the variabledisplacement hydraulic motor (9) from a minimum capacity position to amaximum capacity position by moving between both stroke end positions;

one or two or more restricting members (12, 67) which restrict themovement of the piston (11) at one or two or more middle positions bybeing positioned at one or two or more middle positions between both thestroke end positions; and

position control means which changes the rotation speeds of the rotatingbodies among three or more levels by the piston (11) and the restrictingmembers (12, 67).

The eleventh invention will be specifically described with reference toFIG. 17.

The first piston 11 moves between the stroke end position shown in FIG.17(a) and the stroke end position shown in FIG. 17(d). With the movementof the first piston 11 between both stroke end positions, the capacityposition of the variable displacement hydraulic motor 9 varies from themaximum capacity position to the minimum capacity position.

The third piston 67 is positioned at the first middle position as shownin FIG. 17(b), and the second piston 12 is positioned at the secondmiddle position as shown in FIG. 17(c). When the first piston 11 comesin contact with the third piston 67, the movement of the first piston 11is restricted at the first middle position, and when the first piston 11comes in contact with the second piston 12, the movement of the firstpiston 11 is restricted at the second middle position.

The first piston 11, the second piston 12 and the third piston 67 arediscontinuously positioned at the respective positions.

In other words, when the first piston 11 is positioned at one stroke endposition as shown in FIG. 17(a), the capacity position of the variabledisplacement hydraulic motor 9 becomes the maximum capacity position,and the rotation speed of the rotating body becomes the minimum speed.

And, when the third piston 67 is positioned at the first middle positionand the first piston 11 is positioned to come in contact with the thirdpiston 67 as shown in FIG. 17(b), the capacity position of the variabledisplacement hydraulic motor 9 becomes the first middle capacityposition, and the rotation speed of the rotating body becomes the firstmiddle speed.

And, when the second piston 12 is positioned at the second middleposition and the first piston 11 is positioned to come in contact withthe second piston 12 as shown in FIG. 17(c), the capacity position ofthe variable displacement hydraulic motor 9 becomes the second middlecapacity position, and the rotation speed of the rotating body becomesthe second middle speed.

And, when the first piston 11 is positioned at the other stroke endposition as shown in FIG. 17(d), the capacity position of the variabledisplacement hydraulic motor 9 becomes the minimum capacity position,and the rotation speed of the rotating body becomes the maximum speed.

According to the eleventh invention, the rotation speeds of the rotatingbodies can be switched among three or more speeds by discontinuouslypositioning the first piston 11, the second piston 12 and the thirdpiston 67 at the respective positions. Therefore, the rotation speed ofthe hydraulic motor can be changed among three or more levels by asimple structure and simple control without using a complex structuresuch as a servo valve and complex control.

Because the number of speed changes of the hydraulic motor is increasedto three or more levels, a maximum discharge amount of the hydraulicpump can be made small, and the hydraulic pump can be made small insize.

A twelfth invention relates to the eleventh invention, wherein theposition control means comprises:

respective pressure-receiving chambers (13, 14, 15, 68) which apply thepressure oil to the piston (11) and the one or two or more restrictingmembers (12, 67); and

pressure oil supply means which previously determines combinations ofhigh and low pressures of the pressure oil supplied to the respectivepressure-receiving chambers (13, 14, 15, 68) for the respective rotationspeeds of the rotating bodies and supplies the pressure oil having thecombinations of high and low pressures corresponding to the rotationspeed to be changed to the pressure-receiving chambers (13, 14, 15, 68)respectively.

The twelfth invention will be described with reference to FIG. 17 andFIG. 18.

The first piston 11, the second piston 12 and the third piston 67 aremoved when the pressure oil acts on the pressure-receiving chambers 13,14, 15, 68.

As shown in FIG. 18, the combinations of high and low (ON, OFF)pressures of the pressure oil supplied to the respectivepressure-receiving chambers 13, 14, 15, 68 are previously determined foreach rotation speed of the rotating body. When the pressure oils havingthe combinations of the high and low pressures corresponding to therotation speed to be changed are supplied to the respectivepressure-receiving chambers 13, 14, 15, 68, the first piston 11, thesecond piston 12 and the third piston 67 are discontinuously positioned,and the rotation speed of the rotating body is changed.

According to the twelfth invention, the pressure oils having thecombination of high and low pressures corresponding to the rotationspeed to which the speed is changed are supplied to the respectivepressure-receiving chambers 13, 14, 15, 68 to discontinuously positionthe first piston 11, the second piston 12 and the third piston 67, sothat the hydraulic motor structure and control can be facilitatedfurther more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional diagram showing a positioning device having amiddle capacity adjusting mechanism in its body, which is in a maximumcapacity state;

FIG. 2 is a sectional diagram showing the positioning device having themiddle capacity adjusting mechanism in its body, which is in a middlecapacity state;

FIG. 3 is a sectional diagram showing the positioning device having themiddle capacity adjusting mechanism in its body, which is in a minimumcapacity state;

FIG. 4 is a sectional diagram of inclined shaft type axial piston motor9, which is in a maximum capacity state;

FIG. 5 is a sectional diagram of the inclined shaft type axial pistonmotor 9, which is in a middle capacity state;

FIG. 6 is a sectional diagram of the inclined shaft type axial pistonmotor 9, which is in a minimum capacity state;

FIG. 7 is a diagram showing the relation between ON/OFF states ofsupplying pressure oil to first, second and third pressure-receivingchambers and the capacities of hydraulic motors;

FIG. 8 is a sectional diagram of a conventional hydraulic motor;

FIG. 9 is a sectional diagram of the conventional hydraulic motor;

FIG. 10 is a sectional diagram of the conventional hydraulic motor;

FIGS. 11(a) and 11(b) are diagrams showing comparison between atwo-stage change and a three-stage change of the capacity of a hydraulicmotor;

FIG. 12 is a sectional diagram showing a positioning device havingrespective capacity adjusting mechanisms mounted in its body;

FIG. 13 is an oil hydraulic circuit chart of the present embodiment;

FIG. 14 is an oil hydraulic circuit chart of another embodimentdifferent from FIG. 13;

FIGS. 15(a), 15(b) and 15(c) are diagrams showing the relation amongON/OFF states of supplying the pressure oil to first, second and thirdpressure-receiving chambers, the capacities of a hydraulic motor, andthe switching states of a selector switch;

FIG. 16 is a sectional diagram showing a positioning device which canmake positioning at four positions;

FIG. 17(a) is a diagram showing a maximum capacity state, FIG. 17(b) isa diagram showing a first medium capacity state, FIG. 17(c) is a diagramshowing a second medium capacity state, and FIG. 17(d) is a diagramshowing a minimum capacity state;

FIG. 18 is a diagram showing the relation between the supply of pressureoil to first, second, third and fourth pressure-receiving chambers andthe capacities of variable displacement hydraulic motor 9;

FIG. 19 is a sectional diagram showing a positioning device which canmake positioning at six positions;

FIG. 20 is an oil hydraulic circuit chart of a motor mechanism which canchange the speed among four speeds; and

FIG. 21 is a diagram showing the relation among ON/OFF states ofsupplying the pressure oil to first, second, third and fourthpressure-receiving chambers, the capacities of a hydraulic motor and theswitching states of a switch.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of a positioning device according to the presentinvention, a capacity controller for a variable displacement pistonmotor/pump using the positioning device, and a speed changing device ofrotating bodies using a hydraulic motor will be described with referenceto the accompanying drawings.

In this embodiment, a variable displacement piston motor using apositioning device which can fix at three positions is assumed.

FIG. 1 is a sectional diagram of a positioning device.

A structure of the positioning device used in the invention will bedescribed with reference to FIG. 1.

A positioning device 45 comprises a body 44 and a cover 41. A hole 70 isformed in the body 44. The hole 70 has the same diameter along any partof it.

A first piston 11 and a second piston 12 are placed in the hole 70 ofthe body 44 with their end faces 11 a, 12 a opposed to each other. Thefirst piston 11 and the second piston 12 are slidable in the hole 70 inits longitudinal direction. The first piston 11 and the second piston 12have the same outside diameter. The first piston 11 and the secondpiston 12 are designed to have the same diameter at any parts in alongitudinal direction of the pistons. A pin 43 is fixed to the firstpiston 11 by a screw 71. A member to be positioned is pivotablyconnected to the leading end of the pin 43.

The second piston 12 has the end face 12 a opposed to the first piston11 and an end face 12 b on the opposite side of the end face 12 a. Theend face 12 b is opposed to the cover 41. The end face 12 b of thesecond piston 12 has a pressure-receiving area S1.

The first piston 11 has the end face 11 a opposed to the second piston12 and an end face 11 b on the opposite side of the end face 11 a. Theend face 11 b is opposed to a hole end face 70 a. A first piston-guidingmember 42 is formed in the hole end face 70 a. A hole 11 c having adiameter corresponding to the outside diameter of the firstpiston-guiding member 42 is formed in the first piston 11. The firstpiston 11 moves within the hole 70 with a sliding surface of the firstpiston-guiding member 42 and a sliding surface of the hole 11 c of thefirst piston 11 mutually slid.

Here, the end face 11 a of the first piston 11 has a pressure-receivingarea S2, and the end face 11 b has a pressure-receiving area S3.

A first pressure-receiving chamber 13 is a pressure-receiving chamberwhich applies a pressure to the end face 12 b of the second piston 12. Asecond pressure-receiving chamber 14 is a pressure-receiving chamberwhich applies a pressure to the end face 11 a of the first piston 11. Athird pressure-receiving chamber 15 is a pressure-receiving chamberwhich applies a pressure to the end face 11 b of the first piston 11.

A middle position adjusting mechanism 40 is fitted to the cover 41. Themiddle position adjusting mechanism 40 comprises an adjusting screw 40 aand a nut 40 c. The bottom end of the adjusting screw 40 a is fixed tothe cover 41 by the nut 40 c. A second piston restriction section 40 bis formed on the leading end of the adjusting screw 40 a. A hole 12 chaving a diameter corresponding to the outside diameter of the adjustingscrew 40 a is formed on the second piston 12. The second piston 12 moveswithin the hole 70 with a sliding surface of the hole 12 c of the secondpiston 12 and a sliding surface of the adjusting screw 40 a mutuallyslid.

The second piston restriction section 40 b decides a stop position ofthe second piston 12 when it is moved toward the first piston 11.

On the other hand, a hole 11 d having a diameter corresponding to theoutside diameter of the second piston restriction section 40 b is formedin the first piston 11. The first piston 11 moves within the hole 70with the sliding surface of the hole 11 d of the first piston 11 and thesliding surface of the second piston restriction section 40 b mutuallyslid.

The middle capacity position is adjusted by the middle positionadjusting mechanism 40. Specifically, a fixed state of the adjustingscrew 40 a by the nut 40 is released to adjust a screw-in amount of theadjusting screw 40 a, and the adjusting screw 40 a is fixed again to thecover 41 by the nut 40 c. Thus, the middle capacity position isadjusted.

FIG. 2 and FIG. 3 are also sectional diagrams of the positioning device.FIG. 1 to FIG. 3 show three positions where the positioning device canfix the member to be positioned.

Then, an embodiment of the capacity controller for the variabledisplacement piston motor/pump will be described with reference to thedrawings.

FIG. 4 is a sectional diagram of an inclined shaft type axial pistonmotor 9.

This piston motor 9 generally comprises a casing 52 which accommodates acylinder block 47, a valve plate 46 and a main shaft 51, and thepositioning device 45 which makes positioning of a capacity position ofthe piston motor 9 among three levels by positioning the pivotingposition of the valve plate 46 in three levels.

The cylinder block 47 accommodates a center shaft 10 and a piston 49.One end of the center shaft 10 is pivotably connected to the main shaft51. Other end of the center shaft 10 is connected to the valve plate 46.The piston 49 is slidably placed in cylinder 48 formed within thecylinder block 47.

The capacity of the piston motor 9 is variable according to pivoting ofthe valve plate 46. Specifically, when the valve plate 46 is pivoted, anangle of the center shaft 10 to the main shaft 51 is changed, and astroke amount of the piston 49 is changed. In other words, a differenceof capacity occurs between the pistons 49 and 49 which are mutuallyopposed with the center shaft 10 between them. The main shaft 51 is anoutput shaft of the hydraulic motor.

A stop position of the valve plate 46 is restricted by a maximumcapacity adjusting mechanism 53 and a minimum capacity adjustingmechanism 54. The minimum capacity adjusting mechanism 54 comprises anadjusting screw 54 b fitted to the casing 52 and a nut 54 a for fixingthe adjusting screw 54 b to the casing 52. One leading end of theadjusting screw 54 b is protruded from the inside surface of the casing52, and the other leading end is protruded from the outside surface ofthe casing 52. The maximum capacity adjusting mechanism 53 also has thesame structure.

The motor has a minimum capacity when the valve plate 46 is in contactwith the leading end of the adjusting screw 54 b protruded from theinside surface of the casing 52. Similarly, the motor has a maximumcapacity when the valve plate 46 is in contact with the leading end ofthe adjusting screw 53 b protruded from the inside surface of the casing52.

The minimum capacity position is adjusted by the minimum capacityadjusting mechanism 54.

Specifically, the fixed state of the adjusting screw 54 b by the nut 54a is released to adjust a screw-in amount of the adjusting screw 54 b,and the adjusting screw 54 b is fixed again to the casing 54 by the nut54 a. Thus, the minimum capacity position is adjusted. Similarly, themaximum capacity position is adjusted.

Then, the control of the capacity of the piston motor 9 will bedescribed with additional reference to FIG. 7.

FIG. 7 is a diagram showing the relation between ON/OFF of the supply ofthe pressure oil to the first pressure-receiving chamber 13, the secondpressure-receiving chamber 14 and the third pressure-receiving chamber15 and the capacities of the hydraulic motor 9. In FIG. 7, “ON”indicates that the pressure oil of a high pressure is supplied to thepressure-receiving chamber, and “OFF” indicates that the pressure oil ofa low pressure is supplied to the pressure-receiving chamber. The lowpressure state means, for example, a state that the supply of thepressure oil is intercepted. For convenience of description, it isassumed that the “high pressure” has magnitude P and the “low pressure”has magnitude O. In FIG. 7, the “Invention” indicates the embodimentsshown in FIG. 4 to FIG. 6, while the “Conventional” means the prior artshown in FIG. 10. States actually used in the embodiments shown in FIG.4 to FIG. 6 are indicated within the heavy lines in FIG. 7.

When the supply of the pressure oil to the respective pressure-receivingchambers 13, 14, 15 is controlled in the combination indicated in thefifth columns from the top and in the heavy line in FIG. 7, the firstpiston 11 and the second piston 12 have the states as shown in FIG. 4.At this point, the valve plate 46 is positioned at the maximum capacityposition.

Specifically, the pressure oil of a high pressure is supplied to thesecond pressure-receiving chamber 14 and the third pressure-receivingchamber 15. At this point, the first piston 11 has a difference ofpressure-receiving surface areas S2−S3(>0) between thepressure-receiving area S2 of the end face 11 a and thepressure-receiving area S3 of the end face 11 b. Therefore, forceF=(S2−S3)·P is applied to the first piston 11 to move toward hole endface 70 a. Therefore, the first piston 11 is moved to a position so thatthe valve plate 46 comes in contact with the leading end of theadjusting screw 53 b for the maximum capacity adjustment. The valveplate 46 comes in contact with the leading end of the adjusting screw 53b for the maximum capacity adjustment before the first piston 11 comesin contact with the hole end face 70 a. Thus, the first piston 11 ispositioned at a position away from the second piston 12. At this point,the valve plate 46 is positioned at the maximum capacity position.

When the supply of the pressure oil to the respective pressure-receivingchambers 13, 14, 15 is controlled in the combination shown in the thirdcolumns indicated by a heavy line in FIG. 7, the first piston 11 and thesecond piston 12 have the state as shown in FIG. 5. At this point, thevalve plate 46 is positioned at the middle capacity position.

Specifically, the pressure oil of a high pressure is supplied to thefirst pressure-receiving chamber 13 and the third pressure-receivingchamber 15. At this point, there is a difference of pressure-receivingsurface areas S1−S3(>0) between the pressure-receiving area S1 of theend face 12 a of the second piston 12 and the pressure-receiving area S3of the end face 11 b of the first piston 11. Therefore, forceF=(S1−S3)·P is applied to the first piston 11 and the second piston 12to move toward the hole end face 70 a. The second piston 12 is contactedto the second piston, restriction section 40 b of the adjusting screw 40a, and the movement of the second piston 12 is restricted. The firstpiston 11 comes in contact with the second piston 12 which has itsmovement restricted by the second piston restriction section 40 b. Thus,the first piston 11 is positioned at the middle position. And, the valveplate 46 is positioned at the middle capacity position.

When the supply of the pressure oil to the respective pressure-receivingchambers 13, 14, 15 is controlled in the combination shown in theseventh columns indicated by a heavy line in FIG. 7, the first piston 11and the second piston 12 have the state as shown in FIG. 6. At thispoint, the valve plate 46 is positioned at the minimum capacityposition.

Specifically, the pressure oil of a high pressure is supplied to thethird pressure-receiving chamber 15. At this point, force F=S3·P acts onthe end face 11 b of the first piston 11 to move toward the secondpiston 12. Therefore, the first piston 11 moves to a position so thatthe valve plate 46 kept in contact with the second piston 12 comes incontact with the leading end of the adjusting screw 54 b for the minimumcapacity adjustment. The valve plate 46 comes in contact with theleading end of the adjusting screw 54 b for the minimum capacityadjustment before the second piston 12 comes in contact with the cover.Thus, the first piston 11 is positioned at a position to come in contactwith the second piston 12 whose movement is not restricted by the secondpiston restriction section 40 b. At this point, the valve plate 46 ispositioned at the minimum capacity position.

The aforesaid embodiment controls the supply of the pressure oil to therespective pressure-receiving chambers 13, 14, 15 in the combinationindicated by the heavy-lined frames. But, the capacity position of thevalve plate 46 can be changed in another combination shown in FIG. 7.

When the supply of the pressure oil to the respective pressure-receivingchambers 13, 14, 15 is controlled in the combinations shown in thefirst, second and sixth heavy line frames instead of the combinationshown in the fifth heavy line frames in FIG. 7, the first piston 11 canalso be positioned in the state shown in FIG. 4, and the capacityposition of the valve plate 46 can be positioned at the maximum capacityposition.

But, when the supply of the pressure oil to the respectivepressure-receiving chambers 13, 14, 15 is controlled in the combinationsshown in the fourth and eighth columns, the position of the first piston11 becomes indefinite, and the capacity position of the valve plate 46becomes indefinite.

Then, operation to adjust the capacity position of the aforesaid pistonmotor 9 will be described.

The valve plate 46 of FIG. 4 is positioned at the maximum capacityposition. At this point, the maximum capacity position is adjusted bythe maximum capacity adjusting mechanism 53.

Specifically, the fixed state of the adjusting screw 53 b by the nut 53a is released to adjust the screw-in amount of the adjusting screw 53 b,and the adjusting screw 53 b is fixed again to the casing 52 by the nut53 a. Thus, the maximum capacity position is adjusted.

The valve plate 46 of FIG. 5 is positioned at the middle capacityposition. Then, the middle capacity position is adjusted by the middleposition adjusting mechanism 40.

Specifically, the fixed state of the adjusting screw 40 a by the nut 40c is released to adjust the screw-in amount of the adjusting screw 40 a,and the adjusting screw 40 a is fixed again to the cover 41 by the nut40 c. Thus, the middle capacity position is adjusted.

The valve plate 46 of FIG. 6 is positioned at the minimum capacityposition. Then, the minimum capacity position is adjusted by the minimumcapacity adjusting mechanism 54.

Specifically, the fixed state of the adjusting screw 54 b by the nut 54a is released to adjust the screw-in amount of the adjusting screw 54 b,and the adjusting screw 54 b is fixed again to the casing 52 by the nut54 a. Thus, the minimum capacity position is adjusted.

The embodiment described above assumes the use as the hydraulic motorfor HST (hydrostatic transmission) vehicle such as bulldozers. In such acase, the main shaft 51 is connected to a running body (a wheel or acaterpillar) of the HST vehicle.

The HST vehicle such as the bulldozer has its right and left runningbodies (wheels or caterpillars) independently driven by the HSTsrespectively mounted on the right and left sides. Specifically, theleft-side running body of the vehicle is independently driven andchanged its speed by the HST exclusive for the left side, namely a pairof hydraulic pump and hydraulic motor. Similarly, the right-side runningbody of the vehicle is independently driven and changed its speed by theHST exclusive for the right side, namely a pair of hydraulic pump andhydraulic motor.

Some HST vehicles change the speed by changing the capacity of thevariable displacement motor. The structures of the variable displacementmotor and pump are shown in FIG. 4 to FIG. 7.

As described above, this embodiment can adjust at the middle capacityposition, so that a difference in the number of rotations between theleft and right hydraulic motors 9 and 9 can be eliminated when thehydraulic motor 9 is changed its speed to the middle capacity position.Therefore, a deviation from the course while running straight can beprevented. Besides, this embodiment changes the capacity position amongthree levels of minimum, maximum and middle capacity positions, so thatthe capacity of the hydraulic pump can be made small, and a size of thehydraulic pump can be made compact accordingly. Thus, hydraulicequipment can be mounted in a limited space.

In addition, the first piston 11 and the second piston 12 are designedto have the same outside diameter in this embodiment, so that they canhave a simple structure, and the hole 70 accommodating the pistons 11,12 is designed to have a simple structure such that it has the samediameter along any part of it. Thus, it has an effect that the workingto produce the positioning device 45 can be facilitated.

Furthermore, according to this embodiment, the position of the firstpiston 11 positioned at the minimum capacity position is adjusted. Theminimum capacity position is susceptible to the hydraulic motor 9.According to this embodiment, the minimum capacity position which islargely different and variable among individuals can be adjusted withease.

According to this embodiment, the middle position adjusting mechanism 40is mounted on the cover 41 of the body 44 along the moving direction ofthe first piston 11 and the second piston 12. Accordingly, the adjustingwork can be performed readily.

Then, the positioning device according to an embodiment different fromthe positioning device according to the embodiment shown in FIG. 1 toFIG. 3 will be described with reference to FIG. 12. Like referencenumerals are used for the like components of the embodiment shown inFIG. 1 to FIG. 3. and their descriptions are omitted.

Differences between the positioning device shown in FIG. 12 and thepositioning device shown in FIG. 1 to FIG. 3 are a maximum positionadjusting mechanism 61 and a minimum position adjusting mechanism 63which are disposed along the moving direction of the first piston 11 andthe second piston 12. And, a middle position adjusting mechanism 62 ismounted on the minimum position adjusting mechanism 63.

Specifically, a hole 90 is formed in a body 44. The first piston 11 andthe second piston 12 are disposed in the hole 90 of the body 44 with theend faces 11 a, 12 a mutually opposed. A cover 64 is fitted to the body44 to oppose the end face 11 b of the first piston 11. A cover 65 isfitted to the body 44 to oppose the end face 12 b of the second piston12. This cover 65 corresponds to the cover 41 of FIG. 1 to FIG. 3.

The maximum position adjusting mechanism 61 is fitted to the cover 64.The maximum position adjusting mechanism 61 comprises an adjusting screw61 a and a nut 61 c. The base end of the adjusting screw 61 a is fixedto the cover 64 by the nut 61 c. A first piston restricting section 61 bis formed on the leading end of the adjusting screw 61 a. The adjustingscrew 61 a is fitted to the same axis as the first piston 11.

The minimum position adjusting mechanism 63 is fitted to the cover 65.The minimum position adjusting mechanism 63 comprises an adjusting screw63 a and a nut 63 c. The base end of the adjusting screw 63 a is fixedto the cover 65 by the nut 63 c. Second piston restricting section 63 bis formed on the leading end of the adjusting screw 63 a. The adjustingscrew 63 a is fitted to the same axis as the second piston 12.

The middle position adjusting mechanism 62 is disposed on the adjustingscrew 63 a of the minimum position adjusting mechanism 63. The middleposition adjusting mechanism 62 comprises an adjusting screw 62 a and anut 62 c. The base end of the adjusting screw 62 a is fixed to the baseend of the adjusting screw 63 a of the minimum position adjustingmechanism 63 by the nut 62 c. A second piston restricting section 62 bis formed on the leading end of the adjusting screw 62 a. A hole 63 dhaving a diameter corresponding to the outside diameter of the adjustingscrew 62 a is formed on the adjusting screw 63 a, and the adjustingscrew 62 a slides within the hole 63 d.

A hole 12 c having a diameter corresponding to the outside diameter ofthe adjusting screw 63 a is formed in the second piston 12, and hole 12d having a diameter corresponding to the outside diameter of the secondpiston restriction section 62 b is formed. The second piston 12 moves inthe hole 90 while the hole 12 c slides in contact with the adjustingscrew 63 a and the hole 12 d slides in contact with the second pistonrestricting section 62 b.

The second piston restricting section 62 b decides a stop position whenthe second piston moves toward the first piston 11.

Operation of a positioning device 66 shown in FIG. 12 will be described.

The maximum capacity position is adjusted by the maximum positionadjusting mechanism 61.

Specifically, the fixed state of the adjusting screw 61 a by the nut 61c is released to adjust the screw-in amount of the adjusting screw 61 a,and the adjusting screw 61 a is fixed again to the cover 64 by the nut61 c. Thus, the maximum capacity position is adjusted.

The minimum capacity position is adjusted by the minimum positionadjusting mechanism 63.

Specifically, the fixed state of the adjusting screw 63 a by the nut 63c is released to adjust the screw-in amount of the adjusting screw 63 a,and the adjusting screw 63 a is fixed again to the cover 65 by the nut63 c. Thus, the minimum capacity position is adjusted.

The middle capacity position is adjusted by the middle positionadjusting mechanism 62.

Specifically, the fixed state of the adjusting screw 62 a by the nut 62c is released to adjust the screw-in amount of the adjusting screw 62 a,and the adjusting screw 62 a is fixed again to the base end of theadjusting screw 63 a of the minimum position adjusting mechanism 63 bythe nut 62 c. Thus, the middle capacity position is adjusted.

In the embodiment shown in FIG. 12, the minimum position adjustingmechanism 63 and the maximum position adjusting mechanism 61 are alsodisposed in addition to the middle position adjusting mechanism 62 onthe body 44 in the moving direction of the first piston 11 and thesecond piston 12. Accordingly, the adjusting work can be performed withease as compared with the minimum capacity adjusting mechanism 54 andthe maximum capacity adjusting mechanism 53 which are disposed in thepivoting direction of the valve plate 46 as shown in FIG. 4. Theembodiment shown in FIG. 12 is suitable when a space enough for themechanism to adjust the capacity is not available around the casing 52.

The maximum capacity adjusting mechanism, the middle capacity adjustingmechanism and the minimum capacity adjusting mechanism shown in FIG. 4to FIG. 6 and FIG. 12 use the adjusting screw and the nut to adjust thecapacity position. But, such arrangement is not limitative but aneccentric cam, shim and electromagnetic solenoid may be used to adjustthe position, that the movement of the valve plate 46, the first piston11 or the second piston 12 is restricted, so to adjust the capacityposition.

The embodiment shown in FIG. 4 to FIG. 6 assumes an inclined shaft typeaxial piston motor (pump). But, a swash plate type motor or pump or aradial type motor or pump may be used in the present invention.

FIG. 13 is an oil hydraulic circuit chart of the HST vehicle in whichthe aforesaid hydraulic motor is installed.

In FIG. 13, like reference numerals are used for the like components asshown in FIG. 1 to FIG. 6 and FIG. 12, and their descriptions areomitted.

As shown in FIG. 13, running bodies 100, 100′ of wheels or caterpillarsare respectively mounted on left and right sides of the HST vehicle.This embodiment assumes the caterpillars as the running bodies. The leftcaterpillar 100 is driven to rotate by the hydraulic motor 9 describedwith reference to FIG. 4 to FIG. 6. Similarly, the right caterpillar100′ is driven to rotate by the hydraulic motor 9′ similar to thehydraulic motor 9. In other words, the left and right caterpillars 100,100′ are rotating bodies which are driven to rotate by the left andright hydraulic motors 9, 9′.

A left motor mechanism 8 is mainly comprised of the left hydraulic motor9. And, right motor mechanism 8′ is mainly comprised of the righthydraulic motor 9′.

The left hydraulic motor 9 is driven by a left hydraulic pump 3 as adrive source. The right hydraulic motor 9′ is driven by a righthydraulic pump 3′ as a drive source. A left pump mechanism 7 is mainlycomprised of the left hydraulic pump 3. And, a right pump mechanism 7′is mainly comprised of the right hydraulic pump 3′.

Specifically, the left-side caterpillar 100 of the vehicle body isindependently driven and changed its speed by HST exclusive for the leftside, namely a pair of the left pump mechanism 7 and the left motormechanism 8. Similarly, the right-side caterpillar 100′ of the vehiclebody is independently driven and changed its speed by HST exclusive forthe right side, namely a pair of the right pump mechanism 7′ and theright motor mechanism 8′.

The HST vehicle is changed its speed by changing the capacities of theleft and right hydraulic motors 9, 9′. The capacities of the left andright hydraulic motors 9, 9′ are changed by a motor capacity switchingmechanism 22.

Now, among the left motor mechanism 8, the right motor mechanism 8′, theleft pump mechanism 7 and the right pump mechanism 7′, the left-sidemechanism will be described as a representative example. A dash (′) isadded to the reference numerals for the component parts of the left-sidemechanism to indicate the components of the right-side mechanism, andtheir descriptions will be omitted if not necessary.

The left hydraulic pump 3 is connected to a rotation shaft 2 as theoutput shaft of engine 1. In other words, the left hydraulic pump 3 is avariable displacement hydraulic pump driven by the engine 1. Thishydraulic pump 3 is, for example, a swash plate type hydraulic pump. Aswash plate 4 of the hydraulic pump 3 has a tilting angle which isvariable depending on the moved position of a servo piston 5. The lefthydraulic pump 3 has two discharge ports 3 a, 3 b. In other words, theleft hydraulic pump 3 is a hydraulic pump which can flow the pressureoil in two directions. When the servo piston 5 changes its position andthe tilting angle of the swash plate 4 is also changed, the dischargedirection of the pressure oil is switched to the discharge port 3 a or 3b, and the capacity of the left hydraulic pump 3 is changed accordingly.

A fixed displacement hydraulic pump 6 is connected to the rotation shaft2 of the engine 1 and driven by the engine 1. The hydraulic pump 6 is asupply source for supplying the original pressure of a pilot pressureoil to a pilot valve 23. The servo piston 5 is supplied with the pilotpressure oil controlled by the pilot valve 23.

The discharge port 3 a on one side of the left hydraulic pump 3 iscommunicated with a port 9 a on one side of the left hydraulic motor 9through a pipe 25. The discharge port 3 b on the other side of the lefthydraulic pump 3 is communicated with a port 9 b on the other side ofthe left hydraulic motor 9 through a pipe 26.

Therefore, when the servo piston 5 is supplied with a pilot pressure oilfrom the pilot valve 23, the tilting angle of the swash plate 4 of theleft hydraulic pump 3 is changed. Thus, the discharge port through whichthe pressure oil is discharged from the left hydraulic pump 3 isswitched between 3 a and 3 b, and the pressure oil is flown into theport 9 a or 9 b of the left hydraulic motor 9.

When the pressure oil is flown into the port 9 a of the left hydraulicmotor 9, the left hydraulic motor 9 is rotated in one direction (calledthe normal direction), and the left caterpillar 100 is driven in thenormal direction (called the forward direction). In this case, thepressure oil is flown from the port 9 b on the opposite side of the port9 a of the left hydraulic motor 9 to the pipe 26 and circulated to thevariable displacement hydraulic pump 3.

And, when the pressure oil is flown into the port 9 b of the lefthydraulic motor 9, the left hydraulic motor 9 is rotated in anotherdirection (called the opposite direction), and the left caterpillar 100is driven in the opposite direction (called the reverse direction). Inthis case, the pressure oil is flown from the port 9 a on the oppositeside of the port 9 b of the left hydraulic motor 9 to the pipe 25 andcirculated to the variable displacement hydraulic pump 3.

A right hydraulic pump 8′ and the right hydraulic motor 9′ also operatein the same way as described above.

Then, the structure of the motor capacity switching mechanism 22 will bedescribed.

The motor capacity switching mechanism 22 comprises a three-positionselector valve 20, a selector valve controller 17, pressure sensors 18,19 and a high-pressure selecting valve 21.

The three-position selector valve 20 is supplied with an originalpressure through a pipe 104.

Specifically, the left motor mechanism 8 is provided with check valves101, 102. A pressure oil inlet port of the check valve 101 is connectedto the pipe 25. A pressure oil inlet port of the check valve 102 isconnected to the pipe 26. Pressure oil outlet ports of the check valves101, 102 are connected to a pipe 103. The pipe 103 is connected to aninlet port of the high-pressure selection valve 21. Similarly, checkvalves 101 ′, 102′ are disposed on the right motor mechanism 8′. Apressure oil inlet port of the check valve 101′ is connected to a pipe25′. A pressure oil inlet port of the check valve 102′ is connected to apipe 26′. Pressure oil outlet ports of the check valves 101′, 102′ areconnected to a pipe 103′. The pipe 103′ is connected to an inlet port ofthe high-pressure selection valve 21. An outlet port of thehigh-pressure selection valve 21 is connected to a pipe 104.

Therefore, between a pressure of the pressure oil in the pipe 25 and apressure of the pressure oil in the pipe 26, the pressure oil having ahigher pressure is flown to the pipe 103 through the check valves 101,102. And, between a pressure of the pressure oil in the pipe 25′ and apressure of the pressure oil in the pipe 26′, the pressure oil having ahigher pressure is flown to the pipe 103′ through the check valves 101′,102′. In this embodiment, the two check valves 101, 102 or the checkvalves 101′, 102′ are used, but a shuttle valve may be used instead ofthe two check valves.

The high-pressure selection valve 21 selects the pressure oil having ahigher pressure between a pressure of the pressure oil in the pipe 103and a pressure of the pressure oil in the pipe 103′ to flow to the pipe104.

Thus, the pressure oil having a higher pressure between the pressureoils flown into the left and right hydraulic motors 9, 9′ is supplied asthe original pressure to a 3-position selector valve 20 (FIG. 13 andFIG. 14).

As described above, in the embodiment illustrated, the pressure oilselected by the high-pressure selection valve 21 acts on the left andright motor mechanisms 8, 8′ through the single 3-position selectorvalve 20.

Here, an oil hydraulic circuit for operating the left and right motormechanisms 8, 8′ may be configured as follows. The pressure oil in thepipe 103 drives only the left motor mechanism 8 through a single3-position selector valve, and the pressure oil in the pipe 103′ drivesonly the right motor mechanism 8′ through another 3-position selectorvalve. In this case, the two 3-position selector valves aresimultaneously controlled by the selector valve controller 17 byswitching the selector switch 16.

The pressure oil (original pressure of the 3-position selector valve) inthe pipe 104 is detected by the pressure sensors 18, 19 disposed on thepipe 104. The pressure sensor 18 is a sensor which detects that apressure of the pressure oil in the pipe 104 becomes a first thresholdor higher. The pressure sensor 19 is a sensor which detects that apressure of the pressure oil in the pipe 104 has a second threshold orhigher. Here, the second threshold is greater than the first threshold.

Detection signals output from the pressure sensors 18, 19 are enteredthe selector valve controller 17.

The 3-position selector valve 20 has three valve positions, namely amaximum position 20 a, a middle position 20 b and a minimum position 20c. Pipes 27 a, 28 a are connected to the 3-position selector valve 20.The pipe 27 a is branched to left and right pipes 27, 27′. The pipe 27is communicated with first pressure-receiving chamber 13 of the leftmotor mechanism 8. Similarly, the pipe 27′ is communicated with a firstpressure-receiving chamber 13′ of the right motor mechanism 8′. The pipe28 a is branched to left and right pipes 28, 28′. The pipe 28 iscommunicated with a second pressure-receiving chamber 14 of the leftmotor mechanism 8. Similarly, the pipe 28′ is communicated with a secondpressure-receiving chamber 14′ of the right motor mechanism 8′.

The pipe 104 is branched to left and right pilot pipes 29, 29′. The pipe29 is communicated with a third pressure-receiving chamber 15 of theleft motor mechanism 8. Similarly, the pipe 29′ is communicated with athird pressure-receiving chamber 15′ of the right motor mechanism 8′.Therefore, the respective pressure-receiving chambers 13 to 15 and 13′to 15′ of the left and right motor mechanisms 8, 8′ are supplied with asignal pressure having the higher pressure as the original pressurebetween the pressure oils flown into the left and right hydraulic motors9, 9′.

Therefore, when the valve position of the 3-position selector valve 20is switched to the maximum position 20 a, the capacities of thehydraulic motors 9, 9′ are positioned at the maximum capacity position.When the valve position of the 3position selector valve 20 is switchedto the middle position 20 b, the capacities of the hydraulic motors 9,9′ are positioned at the middle capacity position. And, when the valveposition of the 3-position selector valve 20 is switched to the minimumposition 20 c, the capacities of the hydraulic motors 9, 9′ arepositioned at the minimum capacity position.

Switching of the 3-position selector valve 20 is controlled by theselector valve controller 17. A control signal output from the selectorvalve controller 17 is applied to the electromagnetic solenoid of the3-position selector valve 20, and the valve position of the 3-positionselector valve 20 is switched (the circuit chart of FIG. 13). Thecontrol signal output from the selector valve controller 17 may be usedas a hydraulic pilot pressure instead of an electric current to act onthe 3-position selector valve 20 so to switch the valve position of the3-position selector valve 20.

The selector valve controller 17 is provided with selector switch 16which is manually operated to change the speed of the left and righthydraulic motors 9, 9′. When the selector switch 16 is switched to “Lo”side, a Lo signal for changing the left and right hydraulic motors 9, 9′to the low rotations is entered the selector valve controller 17. And,when the selector switch 16 is switched to “Hi” side, a Hi signal forchanging the left and right hydraulic motors 9, 9′ to the high rotationsis entered the selector valve controller 17.

Now, the contents of control made by the selector valve controller 17will be described with reference to FIG. 15. Operation of the lefthydraulic motor 9 will be typically described below.

FIG. 15(b) is a diagram showing relations among ON/OFF of the supply ofpressure oil to the first, second and third pressure-receiving chambers13, 14, 15, the capacity of the hydraulic motor and the switchingcondition of the selector switch 16. In the drawing, “ON” indicates thatthe pressure oil of a high pressure is being supplied to thepressure-receiving chamber, and “OFF” indicates that the pressure oil ofa low pressure is being supplied to the pressure-receiving chamber. Thelow pressure condition is, for example, a state that the supply of thepressure is stopped. In the following description, a magnitude of the“high pressure” is P and a magnitude of the “low pressure” is 0 (gagepressure) for convenience of description. In the drawing, thecombination (Max., OFF, ON, ON) in the first columns corresponds to thecombination in the fifth heavy-lined columns of FIG. 7. The combination(Intermediate, ON, OFF, ON) in the second columns correspond to thecombination in the third heavy-lined columns of FIG. 7. The combination(Minimum, OFF, OFF, ON) in the third columns corresponds to the seventhheavy-lined columns in FIG. 7.

When the selector switch 16 is switched to “Lo”, a Lo signal forchanging the speed of the left hydraulic motor 9 to the low rotation isentered the selector valve controller 17. Then, the selector valvecontroller 17 applies a control signal for switching the valve positionof the 3-position selector valve 20 to the maximum position 20 a to theelectromagnetic solenoid of the 3-position selector valve 20. Thus, thevalve position of the 3-position selector valve 20 is switched to themaximum position 20 a.

Therefore, the pressure oil of high pressure P is supplied to the secondpressure-receiving chamber 14 from the 3-position selector valve 20through the pipes 28 a, 28. And, the pressure oil of high pressure P issupplied to the third pressure-receiving chamber 15 through the pipe 104and the pilot pipe 29. The first pressure-receiving chamber 13 iscommunicated with tank 24 through the pipes 27, 27 a and the 3-positionselector valve 20, and the pressure oil in the first pressure-receivingchamber 13 is in a low pressure state.

As a result, the first piston 11 and the second piston 12 are in a statepositioned at the maximum capacity position shown in FIG. 4. At thispoint, a center shaft 10 of the left hydraulic motor 9 is changed tohave an angle corresponding to the maximum capacity position.

Similarly, a center shaft 10′ of the right hydraulic motor 9′ is changedto have an angle corresponding to the maximum capacity position.

Thus, the capacity positions of the left and right hydraulic motors 9,9′ are switched to the maximum capacity position, and the left and righthydraulic motors 9, 9′ are changed their speeds to the first speed of alow rotation speed with high torque. The left and right caterpillars100, 100′ are changed their rotation speeds to the minimum rotationspeed.

At this time, the left and right hydraulic motors 9, 9′ are adjusted tohave the same maximum capacity position by the maximum capacityadjusting mechanism 53 shown in FIG. 4. Thus, the left and rightcaterpillars 100, 100′ are controlled to have the same rotation speed.

When the selector switch 16 is switched to “Hi”, a Hi signal forchanging the speed of the left hydraulic motor 9 to the high rotation isentered the selector valve controller 17. Then, the selector valvecontroller 17 applies to the electromagnetic solenoid of the 3-positionselector valve 20 a control signal for switching the valve position ofthe 3-position selector valve 20 to the middle position 20 b or theminimum position 20 c according to the detection signals of the pressuresensors 18, 19. Thus, the valve position of the 3-position selectorvalve 20 is changed to the middle position 20 b or the minimum position20 c.

The selector valve controller 17 has a timer therein to count that thepressure oil in the pipe 104 has a pressure of the aforesaid secondthreshold or higher continuously for one second or more according to thepressure sensor 19 or the pressure oil in the pipe 104 has a pressure ofthe aforesaid first threshold or below continuously for one second ormore according to the pressure sensor 18.

When it is counted by the aforesaid timer that the pressure oil in thepipe 104 has a pressure of the aforesaid second threshold or highercontinuously for one second or more, a control signal for switching tothe middle position 20 b is applied from the selector valve controller17 to the electromagnetic solenoid of the 3-position selector valve 20.Thus, the valve position of the 3-position selector valve 20 is switchedto the middle position 20 b.

Therefore, the pressure oil of high pressure P is supplied from the3-position selector valve 20 to the first pressure-receiving chamber 13through the pipes 27 a, 27. The pressure oil of high pressure P issupplied to the third pressure-receiving chamber 15 through the pipe 104and the pilot pipe 29. The second pressure-receiving chamber 14 iscommunicated with the tank 24 through the pipes 28, 28 a and the3-position selector valve 20, and the pressure oil in the secondpressure-receiving chamber 14 has a low pressure.

As a result, the first piston 11 and the second piston 12 are in a statepositioned at the middle capacity position shown in FIG. 5. At thispoint, the center shaft 10 of the left hydraulic motor 9 is changed tohave an angle corresponding to the middle capacity position.

Similarly, the center shaft 10′ of the right hydraulic motor 9′ ischanged to have an angle corresponding to the middle capacity position.

Thus, the capacity positions of the left and right hydraulic motors 9,9′ are changed to the middle capacity position, and the left and righthydraulic motors 9, 9′ are changed their speeds to the second speed atthe middle rotation speed with intermediate torque. The rotation speedsof the left and right caterpillars 100, 100′ are changed to the middlerotation speed.

At this point, the left and right hydraulic motors 9, 9′ are adjusted tohave the same middle capacity position by the middle position adjustingmechanism 40 shown in FIG. 5. Thus, the left and right caterpillars 100,100′ can be adjusted to have the same rotation speed.

When it is counted by the aforesaid timer that the pressure oil in thepipe 104 has a pressure of the aforesaid first threshold or belowcontinuously for one second or more, a control signal for switching tothe minimum position 20 c is applied from the selector valve controller17 to the electromagnetic solenoid of the 3-position selector valve 20.Thus, the valve position of the 3-position selector valve 20 is switchedto the minimum position 20 c.

Therefore, the pressure oil of high pressure P is supplied to the thirdpressure-receiving chamber 15 through the pipe 104 and the pilot pipe29. The first pressure-receiving chamber 13 is communicated with thetank 24 through the pipes 27, 27 a and the 3-position selector valve 20,and the hydraulic oil in the first pressure-receiving chamber 13 is in alow pressure state. The second pressure-receiving chamber 14 iscommunicated with the tank 24 through the pipes 28, 28 a and the3-position selector valve 20, and the pressure oil in the secondpressure-receiving chamber 14 is in a low pressure state.

As a result, the first piston 11 and the second piston 12 are positionedat the minimum capacity position shown in FIG. 6. At this point, thecenter shaft 10 of the left hydraulic motor 9 is changed to have anangle corresponding to the minimum capacity position.

Similarly, the center shaft 10′ of the right hydraulic motor 9′ ischanged to have an angle corresponding to the minimum capacity position.

Thus, the capacity positions of the left and right hydraulic motors 9,9′ are changed to the minimum capacity position, and the left and righthydraulic motors 9, 9′ are changed their speeds to the third speed at ahigh rotation speed with low torque. The left and right caterpillars100, 100′ are changed their rotation speeds to the maximum rotationspeed.

At this point, the left and right hydraulic motors 9, 9′ are adjusted tohave the same minimum capacity position by the minimum capacityadjusting mechanism 54 shown in FIG. 6. Thus, the left and rightcaterpillars 100, 100′ can be adjusted to have the same rotation speed.

As described above, when the hydraulic motors 9, 9′ are used to changethe rotation speeds of the left and right caterpillars 100, 100′ amongthree levels in this embodiment, a difference in the number of rotationsbetween the left and right caterpillars 100, 100′ can be eliminated toprevent a deviation from the course.

And, according to this embodiment, the capacity positions of the leftand right hydraulic motors 9, 9′ are changed among the three levels, sothat the capacities of the left and right hydraulic pumps 3, 3′ can bemade small, and the hydraulic pumps 3, 3′ can be made compact in size.Therefore, the cost of the hydraulic equipment can be reduced, and thehydraulic equipment can be installed in a limited space. Because thehydraulic pumps 3, 3′ and the hydraulic motors 9, 9′ can be used underthe pressure and capacity conditions good for their efficiency, theperformance of the HST vehicle can be improved.

And, according to this embodiment, when the selector switch 16 isswitched to Hi, switching between the second speed and the third speedcan be made automatically. A range between the second speed and thethird speed is a running range at a high rotation speed with low torqueas shown in FIG. 11(b). According to this embodiment, the manual speedchange in the running region requiring the rotation speed is notnecessary, and the operability in the running region can be improved.

Besides, this embodiment provides the following effects.

Specifically, when it is assumed that a difference in rotations of theleft and right hydraulic motors 9, 9′ is ΔNm, time required to changethe speed is Δt, a difference in running length of the left and rightcaterpillars 100, 100′ is ΔL, and a constant defined according to thestructures of the left and right caterpillars 100, 100′ is D, thefollowing relation is established among them.

D·ΔNm·Δt=ΔL  (3)

To prevent a deviation from the course of the vehicle in the automaticspeed change, it is necessary to decrease Δt, and the capacities of thehydraulic motors 9, 9′ during the automatic speed change must be changedin a short duration.

When the capacities of the hydraulic motors 9, 9′ are changed quickly, amotor flow-in pressure in the left pipes 25, 26 and a motor flow-inpressure in the right pipes 25′, 26′ vary largely, resulting in likelyoccurring hunting in controlling. According to this embodiment, when itis counted that the pressure of the pressure oil in the pipe 104 has thesecond threshold or more continuously for one second or more or thepressure of the pressure oil in the pipe 104 is the aforesaid firstthreshold or below continuously for one second or more, the speed ischanged to the second speed or the third speed. Therefore, even when thecapacities of the left and right hydraulic motors 9, 9′ are changed in ashort duration, the motor flow-in pressure in the left pipes 25, 26 andthe motor flow-in pressure in the right pipes 25′, 26′ are preventedfrom varying to suppress hunting in controlling.

In this embodiment, when the switch 16 is operated to the Hi side, theautomatic speed change is performed between the second speed and thethird speed. But, it may be designed to perform the automatic speedchange between the first speed and the second speed by operating theoperation switch 16 to the Lo side as described below.

FIG. 15(c) is a diagram corresponding to FIG. 15(b) of this embodiment.

When the selector switch 16 is switched to the “Lo” side, a Lo signalfor changing the speed of the left hydraulic motor 9 to the lowrotations is entered the selector valve controller 17. Therefore, theselector valve controller 17 applies to the electromagnetic solenoid ofthe 3-position selector valve 20 a control signal for switching thevalve position of the 3-position selector valve 20 to the maximumposition 20 a or the middle position 20 b according to the detectionsignals of the pressure sensors 18, 19. Thus, the valve position of the3-position selector valve 20 is switched to the maximum position 20 a orthe middle position 20 b.

When it is counted by the aforesaid timer that the pressure of thepressure oil in the pipe 104 is the aforesaid second threshold or morecontinuously for one second or more, a control signal for switching tothe maximum position 20 a is applied from the selector valve controller17 to the electromagnetic solenoid of the 3-position selector valve 20.Thus, the valve position of the 3-position selector valve 20 is switchedto the maximum position 20 a.

Therefore, the pressure oil of high pressure P is supplied from the3-position selector valve 20 to the second pressure-receiving chamber 14through the pipes 28 a, 28. And, the pressure oil of high pressure P issupplied to the third pressure-receiving chamber 15 through the pipe 104and the pilot pipe 29. The first pressure-receiving chamber 13 iscommunicated with the tank 24 through the pipes 27, 27 a and the3-position selector valve 20, and the pressure oil in the firstpressure-receiving chamber 13 is in a low pressure state.

As a result, the first piston 11 and the second piston 12 have a statepositioned at the maximum capacity position shown in FIG. 4. At thispoint, the center shaft 10 of the left hydraulic motor 9 is changed tohave an angle corresponding to the maximum capacity position.

Similarly, the center shaft 10′ of the right hydraulic motor 9′ ischanged to have an angle corresponding to the maximum capacity position.

Thus, the capacity positions of the left and right hydraulic motors 9,9′ are changed to the maximum capacity position, and the left and righthydraulic motors 9, 9′ are changed their speeds to have the first speedat a low rotation speed with high torque. The rotation speeds of theleft and right caterpillars 100, 100′ are changed to the minimumrotation speed.

At this point, the left and right hydraulic motors 9, 9′ are adjusted tohave the same maximum capacity position by the maximum capacityadjusting mechanism 53 shown in FIG. 4. Thus, the left and rightcaterpillars 100, 100′ are made to have the same rotation speed.

When it is counted by the aforesaid timer that the pressure of thepressure oil in the pipe 104 is the aforesaid threshold or belowcontinuously for one second or more, a control signal for switching tothe middle position 20 b is applied from the selector valve controller17 to the electromagnetic solenoid of the 3-position selector valve 20.Thus, the valve position of the 3-position selector valve 20 is switchedto the middle position 20 b.

Therefore, the pressure oil of high pressure P is supplied from the3-position selector valve 20 to the first pressure-receiving chamber 13through the pipes 27 a, 27. And, the pressure oil of high pressure P issupplied to the third pressure-receiving chamber 15 through the pipe 104and the pilot pipe 29. The second pressure-receiving chamber 14 iscommunicated with the tank 24 through the pipes 28, 28 a and the3-position selector valve 20, and the pressure of the pressure oil inthe second pressure-receiving chamber 14 is in a low pressure state.

As a result, the first piston 11 and the second piston 12 are in a statepositioned at the middle capacity position as shown in FIG. 5. At thispoint, the center shaft 10 of the left hydraulic motor 9 is changed tohave an angle corresponding to the middle capacity position.

Similarly, the center shaft 10′ of the right hydraulic motor 9′ ischanged to have an angle corresponding to the middle capacity position.

Thus, the capacity positions of the left and right hydraulic motors 9,9′ are switched to the middle capacity positions, and the left and righthydraulic motors 9, 9′ are changed to have the second speed at themiddle rotation speed with intermediate torque. The rotation speeds ofthe left and right caterpillars 100, 100′ are changed to the middlerotation speed.

At this point, the left and right hydraulic motors 9, 9′ are adjusted tohave the same middle capacity position by the middle position adjustingmechanism 40 shown in FIG. 5. Thus, the left and right caterpillars 100,100′ can be made to have the same rotation speed.

When the selector switch 16 is switched to the “Hi” side, a Hi signalfor changing the left hydraulic motor 9 to the high rotation is enteredthe selector valve controller 17. Therefore, the selector valvecontroller 17 applies to the electromagnetic solenoid of the 3-positionselector valve 20 a control signal for switching the valve position ofthe 3-position selector valve 20 to the minimum position 20 c. Thus, thevalve position of the 3-position selector valve 20 is switched to theminimum position 20 c.

Therefore, the pressure oil of high pressure P is supplied to the thirdpressure-receiving chamber 15 through the pipe 104 and the pilot pipe29. The first pressure-receiving chamber 13 is communicated with thetank 24 through the pipes 27, 27 a and the 3-position selector valve 20,and the pressure oil in the first pressure-receiving chamber 13 is in alow pressure state. The second pressure-receiving chamber 14 iscommunicated with the tank 24 through the pipes 28, 28 a and the3-position selector valve 20, and the pressure oil in the secondpressure-receiving chamber 14 is in a low pressure state.

As a result, the first piston 11 and the second piston 12 are in a statepositioned at the minimum capacity position shown in FIG. 6. At thispoint, the center shaft 10 of the left hydraulic motor 9 is changed tohave an angle corresponding to the minimum capacity position.

Similarly, the center shaft 10′ of the right hydraulic motor 9′ ischanged to have an angle corresponding to the minimum capacity position.

Thus, the capacity positions of the left and right hydraulic motors 9,9′ are switched to the minimum capacity position, and the left and righthydraulic motors 9, 9′ are changed their speeds to have the third speedat a high rotation speed with low torque. The rotation speeds of theleft and right caterpillars 100, 100′ are changed to the maximumrotation speed.

At this point, the left and right hydraulic motors 9, 9′ are adjusted tohave the same minimum capacity position by the minimum capacityadjusting mechanism 54 shown in FIG. 6. Thus, the left and rightcaterpillars 100, 100′ can be made to have the same rotation speed.

According to this embodiment, when the selector switch 16 is operatedtoward the Lo side, switching between the first speed and the secondspeed is performed automatically. A range between the first speed andthe second speed is a work range of a low rotation speed with hightorque as shown in FIG. 11(b). According to this embodiment, the manualspeed change in the work range requiring torque can be eliminated, andoperability in the work range can be improved.

In the aforesaid embodiment, when the operation switch 16 is operatedtoward the Hi side, the automatic speed change is performed between thesecond speed and the third speed, and when the operation switch 16 isoperated toward the Lo side, the automatic speed change is performedbetween the first speed and the second speed. But, a switch or the likemay be used to select the automatic switching between the first rotationspeed and the second rotation speed or the automatic switching betweenthe second rotation speed and the third rotation speed. The rangebetween the first rotation speed and the second rotation speed is a workregion of a low rotation speed with high torque. According to thisembodiment, when the automatic speed change in the work region isselected depending on a use condition of the hydraulic motors 9, 9′,operability in the running region is improved, and the speed change inthe running region is freely performed manually. When the automaticspeed change in the running region is selected depending on a usecondition of the hydraulic motors 9, 9′, operability in the runningregion is improved, and the speed change in the work region is freelyperformed manually.

The speed change among the first speed, the second speed and the thirdspeed may be made manually.

FIG. 14 is an oil hydraulic circuit chart related to the manual speedchange among the first speed, the second speed and the third speed. InFIG. 14, like reference numerals are used to indicate the likecomponents of FIG. 13 and their descriptions are omitted. Differencesfrom FIG. 13 will be described below.

Motor capacity switching mechanism 22 of FIG. 14 is different from theone shown in FIG. 13 and not provided with the selector valve controller17. The 3-position selector valve 20 is provided with selector switch 16for manual speed change of left and right hydraulic motors 9, 9′. Whenthe selector switch 16 is switched to “Lo”, a Lo signal for the speedchange of the left and right hydraulic motors 9, 9′ to the first speedis entered the electromagnetic solenoid of the 3-position selector valve20. When the selector switch 16 is switched to “Mid”, a Mid signal forchanging the speeds of the left and right hydraulic motors 9, 9′ to thesecond speed is entered the electromagnetic solenoid of the 3-positionselector valve 20. When the selector switch 16 is switched to “Hi”, a Hisignal. for changing the speeds of the left and right hydraulic motors9, 9′ is entered the electromagnetic solenoid of the 3-position selectorvalve 20.

FIG. 15(a) is a diagram corresponding to FIGS. 15(b), (c) of thisembodiment.

When the selector switch 16 is switched to “Lo”, a Lo signal forchanging the speeds of the left and right hydraulic motors 9, 9′ to thefirst speed is entered the electromagnetic solenoid of the 3-positionselector valve 20. Therefore, the valve position of the 3-positionselector valve 20 is switched to maximum position 20 a, and pressureoils of low pressure (OFF), high pressure (ON) and high pressure (ON)are supplied to first, second and third pressure-receiving chambers ofthe left and right motor mechanisms 8, 8′. Thus, the left and righthydraulic motors 9, 9′ have a state positioned at the maximum capacityposition shown in FIG. 4. At this point, center shafts 10, 10′ of theleft and right hydraulic motors 9, 9′ are changed to have an anglecorresponding to the maximum capacity position.

Thus, the capacity positions of the left and right hydraulic motors 9,9′ are switched to the maximum capacity position, and the left and righthydraulic motors 9, 9′ are changed to have the first speed at a lowrotation speed with high torque. The rotation speeds of left and rightcaterpillars 100, 100′ are switched to the minimum rotation speed.

At this point, the left and right hydraulic motors 9, 9′ are adjusted tohave the same maximum capacity position by the maximum capacityadjusting mechanism 53 shown in FIG. 4. Thus, the left and rightcaterpillars 100, 100′ can be adjusted to have the same rotation speed.

When the selector switch 16 is switched to “Mid”, a Mid signal for thespeed change of the left and right hydraulic motors 9, 9′ to the secondspeed is entered the electromagnetic solenoid of the 3-position selectorvalve 20. Therefore, the valve position of the 3-position selector valve20 is switched to middle position 20 b, and the pressure oils of highpressure (ON), low pressure (OFF) and high pressure (ON) shown in FIG.15(a) are supplied to the first, second and third pressure-receivingchambers of the left and right motor mechanisms 8, 8′. Thus, the leftand right hydraulic motors 9, 9′ have a state positioned at the middlecapacity position shown in FIG. 5. At this point, the center shafts 10,10′ of the left and right hydraulic motors 9, 9′ are changed to have anangle corresponding to the middle capacity position.

Thus, the capacity positions of the left and right hydraulic motors 9,9′ are switched to the middle capacity position, and the left and righthydraulic motors 9, 9′ are changed their speeds to the second speed at amiddle rotation speed with middle torque. The left and rightcaterpillars 100, 100′ are changed their rotation speeds to the middlerotation speed.

At this point, the left and right hydraulic motors 9, 9′ are adjusted tohave the same middle capacity position by the middle position adjustingmechanism 40 shown in FIG. 5. Thus, the left and right caterpillars 100,100′ can be adjusted to have the same rotation speed.

When the selector switch 16 is switched to “Hi”, a Hi signal forchanging the speeds of the left and right hydraulic motors 9, 9′ to thethird speed is entered the electromagnetic solenoid of the 3-positionselector valve 20. Therefore, the valve position of the 3-positionselector valve 20 is switched to minimum position 20 c, and the pressureoils of low pressure (OFF), low pressure (OFF) and high pressure (ON)shown in FIG. 15(a) are supplied to the first, second and thirdpressure-receiving chambers of the left and right motor mechanisms 8,8′. Thus, the left and right hydraulic motors 9, 9′ have a statepositioned at the minimum capacity position shown in FIG. 6. At thispoint, the center shafts 10, 10′ of the left and right hydraulic motors9, 9′ are changed to have an angle corresponding to the minimum capacityposition.

Thus, the capacity positions of the left and right hydraulic motors 9,9′ are switched to the minimum capacity position, and the left and righthydraulic motors 9, 9′ are changed their speeds to have the third speedat the high rotation speed with low torque. The rotation speeds of theleft and right caterpillars 100, 100′ are switched to the maximumrotation speed.

At this point, the left and right hydraulic motors 9, 9′ are adjusted tohave the same minimum capacity position by the minimum capacityadjusting mechanism 54 shown in FIG. 6. Thus, the left and rightcaterpillars 100, 100′ can be adjusted to have the same rotation speed.

According to this embodiment, switching among the first rotation speed,the second rotation speed and the third rotation speed is manuallyperformed by the selector switch 16. When the automatic switching of thecapacities of the hydraulic motors 9, 9′ is not desirable, the switchingcan be made manually as desired.

In this embodiment, the automatic speed change is performed between thefirst speed and the second speed or between the second speed and thethird speed. But, the automatic speed change may be performed among thefirst speed, the second speed and the third speed.

In this embodiment, the automatic speed change is performed according tothe detection signals of the pressure sensors 18, 19. But, the inventionis not limited to the above, and a load on the left and right hydraulicmotors 9, 9′ may be detected by predetermined means to perform theautomatic speed change according to the detected load.

The automatic speed change may be made according to the number ofrevolutions of engine 1. And, the automatic speed change may also beperformed according to the number of rotations of the left and righthydraulic motors 9, 9′.

In this embodiment, the left and right hydraulic motors 9, 9′ areprovided with the hydraulic pumps 3, 3′ respectively. But, a commonhydraulic pump may be mounted for the left and right hydraulic motors 9,9′ to supply the pressure oil discharged from the hydraulic pump to theleft and right hydraulic motors 9, 9′.

This embodiment assumes a running/shifting device for vehicles whichchanges a speed by driving to rotate the left and right running bodies100, 100′ by the left and right hydraulic motors 9, 9′. But, theinvention is not limited to the above and may be applied to any deviceif the speed is changed by driving to rotate the left and right rotatingbodies by the left and right hydraulic motors 9, 9′.

For example, the invention can also be applied to a conveying devicewhich changes its speed by driving to rotate the left and right rotatingbodies by the left and right hydraulic motors.

Then, a positioning device for positioning at four positions, namely themaximum position, the first middle position, the second middle positionand the minimum position, will be described with reference to FIG. 16.

FIG. 16 is a sectional diagram of positioning device 80 for fixing atthe four positions. Like portions of the embodiment shown in FIG. 1 toFIG. 3 will not be described if not necessary.

Differences between the positioning device 80 shown in FIG. 16 and thepositioning device 45 shown in FIG. 1 to FIG. 3 are as follows.Specifically, a third piston 67 is disposed in addition to the firstpiston 11 and the second piston 12. A maximum position adjustingmechanism 81 and a minimum position adjusting mechanism 84 are alsodisposed along the moving direction of the first piston 11, the secondpiston 12 and the third piston 67. A first middle position adjustingmechanism 82 is fitted to the maximum position adjusting mechanism 81,and a second middle position adjusting mechanism 83 is fitted to theminimum position adjusting mechanism 84. Such differences will be mainlydescribed below.

A hole 90 is formed in a body 44. In the hole 90 of the body 44, thefirst piston 11 and the second piston 12 are disposed with end faces 11a, 12 a opposed to each other. The third piston 67 is also disposed inthe hole 90 of the body 44 with an end face 67 b opposed to the end face11 b of the first piston 11. A cover 65 is disposed on the body 44 so tobe opposite to the end face 12 b of the second piston 12. A cover 64 isdisposed on the body 44 so to be opposite to an end face 67 a of thethird piston 67.

The maximum position adjusting mechanism 81 is fitted to the cover 64.The minimum position adjusting mechanism 84 is fitted to the cover 65.The first middle position adjusting mechanism 82 is fitted to themaximum position adjusting mechanism 81. The minimum position adjustingmechanism 84 is fitted to the cover 65. The second middle positionadjusting mechanism 83 is fitted to the minimum position adjustingmechanism 84. The minimum position adjusting mechanism 84 has the samestructure as the maximum position adjusting mechanism 81 does. Thesecond middle position adjusting mechanism 83 has the same structure asthe first middle position adjusting mechanism 82 does. Therefore, thestructures of the maximum position adjusting mechanism 81 and the firstmiddle position adjusting mechanism 82 will be described, while those ofthe minimum position adjusting mechanism 84 and the second middleposition adjusting mechanism 83 will not be described.

Specifically, the maximum position adjusting mechanism 81 comprises anadjusting screw 81 a and a nut 81 c. The adjusting screw 81 a is fixedto the cover 64 by the nut 81 c. The adjusting screw 81 a is installedon the same axis as the third piston 67. Therefore, the adjusting screw81 a can be relatively moved to the cover 64.

The minimum position adjusting mechanism 84 also has the same structureas the maximum position adjusting mechanism 81 does. Therefore, theadjusting screw 84 a can be relatively moved to the cover 65.

The first middle position adjusting mechanism 82 comprises an adjustingscrew 82 a and a nut 82 c. The adjusting screw 82 a is fixed to theadjusting screw 81 a of the maximum position adjusting mechanism 81 bythe nut 82 c. A third piston restriction part 82 b is formed on theadjusting screw 82 a. A hole 81 d having a diameter corresponding to theoutside diameter of the adjusting screw 82 a is formed in the adjustingscrew 81 a of the maximum position adjusting mechanism 81. Therefore,the adjusting screw 82 a can be relatively moved to the adjusting screw81 a.

A hole 67 c having a diameter corresponding to the outside diameter ofthe adjusting screw 82 a is formed in the third piston 67. Therefore,the third piston 67 can be moved along the adjusting screw 82 a. Whenthe third piston 67 moves toward the cover 64, the end face 67 a of thethird piston 67 comes in contact with the adjusting screw 81 a, and thethird piston 67 moves toward the first piston 11. Then, the end face 67b of the third piston 67 comes in contact with the third pistonrestriction part 82 b.

The second middle position adjusting mechanism 83 also has the samestructure as the first middle position adjusting mechanism 82 does.Therefore, an adjusting screw 83 a can be relatively moved to anadjusting screw 84 a. And, the second piston 12 can be moved along theadjusting screw 83 a. When the second piston 12 moves toward the cover65, the end face 12 b of the second piston 12 comes in contact with theadjusting screw 84 a, and the second piston 12 moves toward the firstpiston 11. And, the end face 12 a of the second piston comes in contactwith a second piston restriction part 83 b.

The first pressure-receiving chamber 13 is formed between the adjustingscrew 84 a and the end face 12 b of the second piston 12. The secondpressure-receiving chamber 14 is formed between the end face 12 a of thesecond piston 12 and the end face 11 a of the first piston 11. The thirdpressure-receiving chamber 15 is formed between the end face 11 b of thefirst piston 11 and the end face 67 b of the third piston 67. Fourthpressure-receiving chamber 68 is formed between the end face 67 a of thethird piston 67 and the adjusting screw 81 a.

The first pressure-receiving chamber 13 is a pressure-receiving chamberfor acting a pressure on the end face 12 b of the second piston 12. Thesecond pressure-receiving chamber 14 is a pressure-receiving chamber forapplying a pressure to the end face 12 a of the second piston 12 and theend face 11 a of the first piston 11. The third pressure-receivingchamber 15 is a pressure-receiving chamber for applying a pressure tothe end face 11 a of the first piston 11 and the end face 67 b of thethird piston 67. The fourth pressure-receiving chamber 68 is apressure-receiving chamber for applying a pressure to the end face 67 aof the third piston 67.

Then, the adjusting operations by the adjusting mechanisms 81, 82, 83,84 will be described.

The maximum position of the first piston 11 is adjusted by the maximumposition adjusting mechanism 81.

Specifically, the fixed state of the adjusting screw 81 a by the nut 81c is released to adjust the screw-in amount of the adjusting screw 81 a,and the adjusting screw 81 a is fixed again to the cover 64 by the nut81 c to adjust a relative position of the adjusting screw 81 a to thecover 64. Thus, the maximum position of the first piston 11 can beadjusted.

First middle position of the first piston 11 is adjusted by the firstmiddle position adjusting mechanism 82.

Specifically, the fixed state of the adjusting screw 82 a by the nut 82c is released to adjust the screw-in amount of the adjusting screw 82 a,and the adjusting screw 82 a is fixed again to the adjusting screw 81 aof the maximum position adjusting mechanism 81 by the nut 82 to adjust arelative position of the adjusting screw 82 a to the adjusting screw 81a. Thus, the first middle position of the first piston 11 can beadjusted.

The second middle position of the first piston 11 is adjusted by thesecond middle position adjusting mechanism 83. Its adjusting operationis the same to that of the first middle position adjusting mechanism 82.

And, the minimum position of the first piston 11 is adjusted by theminimum position adjusting mechanism 84. Its adjusting operation is thesame to that of the maximum position adjusting mechanism 81.

Then, a subject to be positioned by the positioning device 80 is assumedto be a valve plate 46 of variable displacement hydraulic motor 9, andcontrol for switching the capacity of the variable displacementhydraulic motor 9 to four levels will be described with reference toFIG. 17 and FIG. 18.

FIG. 17 schematically shows four positioning states of the positioningdevice 80. FIG. 17(a) shows a state that the variable displacementhydraulic motor 9 has the maximum capacity, FIG. 17(b) shows a statethat the variable displacement hydraulic motor 9 has a first middlecapacity, FIG. 17(c) shows a state that the variable displacementhydraulic motor 9 has a second middle capacity, and FIG. 17(d) shows astate that the variable displacement hydraulic motor 9 has a minimumcapacity. The capacity of the variable displacement hydraulic motor 9 isvariable in order of the minimum capacity, the second middle capacity,the first middle capacity and the maximum capacity.

FIG. 18 is a diagram showing a relation between the supply of thepressure oil to the respective pressure-receiving chambers and thecapacity of the variable displacement hydraulic motor 9. In FIG. 18,“ON” and “OFF” are used in the same manner as those of FIG. 7.Specifically, “ON” indicates that the pressure oil of a high pressure issupplied to the pressure-receiving chamber, and “OFF” indicates that thepressure oil of a low pressure is supplied to the pressure-receivingchamber. The low pressure state is, for example, a state that the supplyof the pressure oil is stopped. And, the “high pressure” is indicated byP, and the “low pressure” is indicated by O.

The first piston 11, the second piston 12 and the third piston 67 can bediscontinuously positioned at respective positions by supplying thepressure oil having combinations of high and low pressures shown in FIG.18 to the pressure-receiving chambers 13, 14, 15, 68 respectively.

Specifically, when it is controlled to supply the pressure oil in thecombination shown in the first columns of FIG. 18 to the respectivepressure-receiving chambers 13, 14, 15, 68, the secondpressure-receiving chamber 14 has a high pressure, the third and fourthpressure-receiving chambers 15, 68 have a low pressure, and the firstpiston 11 moves in a direction to push the third piston 67 as shown inFIG. 17(a). As a result, the third piston 67 is positioned at a positionso to come in contact with the adjusting screw 81 a, and the firstpiston 11 is positioned at a position to come into contact with thethird piston 67. Thus, the first piston 11 is positioned at the maximumposition, namely one stroke end position. At this point, first pistonvalve plate 46 connected to the first piston 11 is positioned at themaximum capacity position. Therefore, the capacity position of thevariable displacement hydraulic motor 9 is changed to the maximumcapacity position.

And, the supply of the pressure oil in the combination shown in thesecond columns of FIG. 18 to the respective pressure-receiving chambers13, 14, 15, 68 is controlled, so that the second and fourthpressure-receiving chambers 14, 68 have a high pressure, and the thirdpressure-receiving chamber has a low pressure as shown in FIG. 17(b).There is a pressure-receiving area difference S4−S2(>0) betweenpressure-receiving area S2 of the end face 11 a of the first piston 11and pressure-receiving area S4 of the end face 67 a of the third piston67. Therefore, force F=(S4−S2)·P acts on the first piston 11 and thethird piston 67 to move toward the cover 65. The third piston 67 comesin contact with the third piston restriction part 82 b and its movementis restricted. The first piston 11 comes in contact with the thirdpiston 67. Thus, the third piston 67 is positioned at a position to comeinto contact with the third piston restriction part 82 b, and the firstpiston 11 is positioned at a position to come in contact with the thirdpiston 67. And, the first piston 11 is positioned at the first middleposition. At this point, the first piston valve plate 46 is positionedat the first middle capacity position. Therefore, the capacity positionof the variable displacement hydraulic motor 9 is changed to the firstmiddle capacity position.

And, when it is controlled to supply the pressure oil in the combinationof the third columns of FIG. 18 to the respective pressure-receivingchambers 13, 14, 15, 68, the first and third pressure-receiving chambers13, 15 have a high pressure and the second pressure-receiving chamber 14has a low pressure as shown in FIG. 17(c). There is pressure-receivingarea difference S1−S3(>0) between the pressure-receiving area S3 of theend face 11 b of the first piston 11 and the pressure-receiving area S1of the end face 12 b of the second piston 12. Therefore, forceF=(S1−S3)·P acts on the first piston 11 and the second piston 12 to movetoward the cover 64. The second piston 12 comes in contact with thesecond piston restriction part 83 b and its movement is restricted. Thefirst piston 11 comes in contact with the second piston 12. Thus, thesecond piston 12 is positioned at a position to come into contact withthe second piston restriction part 83 b, and the first piston 11 ispositioned at a position to come into contact with the second piston 12.Thus, the first piston 11 is positioned at the second middle position.At this point, the first piston valve plate 46 is positioned at thesecond middle capacity position. Therefore, the capacity position of thevariable displacement hydraulic motor 9 is changed to the second middlecapacity position.

And, when it is controlled to supply the pressure oil in the combinationof the fourth columns of FIG. 18 to the respective pressure-receivingchambers 13, 14, 15, 68, the third pressure-receiving chamber 15 has ahigh pressure, the first and second pressure-receiving chambers 13, 14have a low pressure, and the first piston 11 moves in a direction topush the second piston 12 as shown in FIG. 17(d). As a result, thesecond piston 12 is positioned at a position to come into contact withthe adjusting screw 84 a, and the first piston 11 is positioned at aposition to come in contact with the second piston 12. Thus, the firstpiston 11 is positioned at the minimum position, namely another strokeend position. At this point, the first piston valve plate 46 ispositioned at the minimum capacity position. Therefore, the capacityposition of the variable displacement hydraulic motor 9 is changed tothe minimum capacity position.

As described above, according to this embodiment, the capacity positionof the hydraulic motor 9 which is subject to positioning can be switchedby discontinuously positioning the first piston 11, the second piston 12and the third piston 67 at the respective positions. Therefore, as shownin FIG. 10, the positioning number of the positioning device can beincreased to three or more by a simple structure and simple control, andthe capacity position of the hydraulic motor can be switched withoutusing the complex structure such as the servo valves and complexcontrol. Therefore, the hydraulic equipment such as the hydraulic pumpcan be made compact in size.

Then, a positioning device 85 which can position at six positions suchas a maximum position, a first middle position, a second middleposition, a third middle position, a fourth middle position and aminimum position will be described with reference to FIG. 19.

FIG. 19(a) is a sectional diagram of the positioning device 85, and FIG.19(b) is a partial sectional diagram taken along line A—A of thepositioning device 85 of FIG. 19(a).

The positioning device 85 shown in FIG. 19 has the first piston 11positioned at the respective positions according to the positionalrelation among the first piston 11, the second piston 12 and the secondpiston restricting part 62 b in the same way as the positioning device66 shown in FIG. 12. But, three pistons 91 to 93 which restrict themovement of the first piston 11 are disposed in addition to the firstpiston 11 and the second piston 12 to freely project into the secondpressure-receiving chamber 14. Differences from the aforesaidpositioning device 66 of FIG. 12 will be mainly described, and the sameportions will be omitted from being described if not necessary.

The body 44 is formed with holes 90 a, 90 b, 90 c as well as hole 90.The holes 90 a, 90 b, 90 c are formed in the body 44 so that theircenter axes are perpendicular to the center axis of the hole 90. Thehole 90 a is formed to be closest to the third pressure-receivingchamber 15, the hole 90 c is formed to be closest to the firstpressure-receiving chamber 13, and the hole 90 b is formed at the middleof them.

The third piston 91 is disposed to be freely slidable in the hole 90 a.A first piston restricting part 91 a is formed on the third piston 91. Afourth pressure-receiving chamber 94 is formed in the hole 90 a andapplies a pressure to the third piston 91. When the fourthpressure-receiving chamber 94 has a high pressure, the third piston 91slides within the hole 90 a, and the first piston restricting part 91 aprotrudes into the second pressure-receiving chamber 14.

Similarly, the fourth piston 92 is disposed to be freely slidable withinthe hole 90 b. The first piston restricting part 92 a is formed on thefourth piston 92. A fifth pressure-receiving chamber 95 is formed withinthe hole 90 b to apply a pressure to the fourth piston 92. When thefifth pressure-receiving chamber 95 has a high pressure, the fourthpiston 92 slides within the hole 90 b, and the first piston restrictingpart 92 a protrudes into the second pressure-receiving chamber 14.

Similarly, the fifth piston 93 is disposed to be freely slidable in thehole 90 c. A first piston restricting part 93 a is formed on the fifthpiston 93. A sixth pressure-receiving chamber 96 is formed within thehole 90 c to apply a pressure to the fifth piston 93. When the sixthpressure-receiving chamber 96 has a high pressure, the fifth piston 93slides within the hole 90 c, and the first piston restricting part 93 aprotrudes into the second pressure-receiving chamber 14.

Then, it is assumed that the subject to be positioned by the positioningdevice 85 is the valve plate 46 of the variable displacement hydraulicmotor 9, and the control to switch the capacity of the variabledisplacement hydraulic motor 9 to six levels will be described. Thecapacity of the variable displacement hydraulic motor 9 varies in thesix levels in order of the minimum capacity, the fourth middle capacity,the third middle capacity, the second middle capacity, the first middlecapacity and the maximum capacity.

The positioning device 85 shown in FIG. 19 can position discontinuouslythe first piston 11, the second piston 12, the third piston 91, thefourth piston 92 and the fifth piston 93 at the respective positions bysupplying the pressure oil having a combination of high and lowpressures to the respective pressure-receiving chambers 13, 14, 15, 94,95, 96.

First, it is assumed that the pressure oil of a low pressure is suppliedto the fourth pressure-receiving chamber 94, the fifthpressure-receiving chamber 95 and the sixth pressure-receiving chamber.At this point, the pressure oil to the first pressure-receiving chamber13, the second pressure-receiving chamber 14 and the thirdpressure-receiving chamber 15 is controlled in the same way as in FIG.7, so that the positioning device 85 operates in the same way as theaforesaid positioning device 45 or 66.

Specifically, the first piston 11 has a state as shown in FIG. 19 andpositioned at the stroke end position away from the second piston 12 bycontrolling the first pressure-receiving chamber 13, the secondpressure-receiving chamber 14 and the third pressure-receiving chamber15 to have a low pressure, a high pressure and a high pressurerespectively. In other words, it is positioned at the maximum position.

And, the first piston 11 is positioned at the fourth middle position tocome in contact with the second piston 12 whose movement is restrictedby the second piston restricting part 62 b by controlling the firstpressure-receiving chamber 13, the second pressure-receiving chamber 14and the third pressure-receiving chamber 15 to have a high pressure, alow pressure and a high pressure respectively.

And, the first piston 11 is positioned at the stroke end position tocome in contact with the second piston 12 whose movement is notrestricted by the second piston restricting part 62 b by controlling thefirst pressure-receiving chamber 13, the second pressure-receivingchamber 14 and the third pressure-receiving chamber 15 to have a lowpressure, a low pressure and a high pressure respectively. In otherwords, it is positioned at the minimum position.

As described above, when the first piston 11 is changed to the maximumposition, the fourth middle position and the minimum position, thevariable displacement hydraulic motor 9 is changed to have the maximumcapacity, the fourth middle capacity and the minimum capacityrespectively.

And, the variable displacement hydraulic motor 9 is changed its capacityto the first middle capacity, the second middle capacity and the thirdmiddle capacity when the first piston 11 is changed to the first middleposition, the second middle position and the third middle position asdescribed below.

When the first piston 11 is to be positioned at the first middleposition, the second middle position or the third middle position, thepressure oil of a low pressure is supplied to the secondpressure-receiving chamber 14, and the pressure oil of a high pressureis supplied to the third pressure-receiving chamber 15. When it iscontrolled so that the second pressure-receiving chamber 14 has a lowpressure and the third pressure-receiving chamber 15 is controlled tohave a high pressure, the first piston 11 moves toward the second piston12.

When the first piston 11 is positioned at the first middle position, thepressure oil of a high pressure is supplied to the fourthpressure-receiving chamber 94, and the pressure oil of a low pressure issupplied to the other fifth and sixth pressure-receiving chambers 95,96. Therefore, the first piston restricting part 91 a of the thirdpiston 91 protrudes into the second pressure-receiving chamber 14 asindicated by a broken line in FIG. 19(b). Thus, the movement of thefirst piston 11 is restricted by the first piston restricting part 91 a,and the first piston 11 is positioned at the first middle position.

Similarly, when the first piston 11 is to be positioned at the secondmiddle position, the pressure oil of a high pressure is supplied to thefifth pressure-receiving chamber 95, and the pressure oil of a lowpressure is supplied to the fourth and sixth pressure-receiving chambers94, 96. Therefore, the first piston restricting part 92 a of the fourthpiston 92 protrudes into the second pressure-receiving chamber 14 asindicated by a broken line in FIG. 19(b). Thus, the movement of thefirst piston 11 is restricted by the first piston restricting part 92 a,and the first piston 11 is positioned at the second middle position.

Similarly, when the first piston 11 is to be positioned at the thirdmiddle position, the pressure oil of a high pressure is supplied to thesixth pressure-receiving chamber 96, and the pressure oil of a lowpressure is supplied to the fourth and fifth pressure-receiving chambers94, 95. Therefore, the first piston restricting part 93 a of the fifthpiston 93 protrudes into the second pressure-receiving chamber 14 asindicated by a broken line in FIG. 19(b). Thus, the movement of thefirst piston 11 is restricted by the first piston restricting part 93 a,and the first piston 11 is positioned at the third middle position.

As described above, according to the positioning device 85 of FIG. 19,the capacity position of the hydraulic motor 9 which is subject topositioning can be switched to the six levels by discontinuouslypositioning the first piston 11, the second piston 12, the third piston91, the fourth piston 92 and the fifth piston 93 at the respectivepositions. By disposing more pistons for restricting the movement of thefirst piston 11, positioning can be made at more positions.

The positioning device 80 and the positioning device 85 shown in FIG. 16and FIG. 19 have the adjusting screw and the nut to configure themaximum capacity adjusting mechanism, the first middle capacityadjusting mechanism, the second middle capacity adjusting mechanism andthe minimum capacity adjusting mechanism so to adjust the capacityposition. But, such a configuration is not limitative, but the positionsto restrict the movement of the first piston 11, the second piston 12and the third piston 67 may be adjusted by an eccentric cam, shim orelectromagnetic solenoid for adjusting the capacity position.

The embodiments shown in FIG. 16 and FIG. 19 may be used for theinclined shaft type axial piston motor (pump) and also be used for aswash plate type motor or pump or a radial type motor or pump.

The aforesaid positioning device 80 can be mounted on the HST vehicle tocontrol its running.

FIG. 20 is an oil hydraulic circuit chart of the HST vehicle whoserunning is controlled by the positioning device 80. A motor mechanism160 shown in FIG. 20 corresponds to the left motor mechanism 8 of FIG.14. The right motor mechanism 8′ is also the same as the motor mechanism160 but its corresponding one is not shown. In FIG. 20, like referencenumerals are used to indicate the like components of FIG. 14 and FIG.16, and their descriptions are omitted. The left hydraulic motor 9 willbe described below as means for changing the speed among a first speed“Lo”, a second speed “Mid1”, a third speed “Mid2” and a fourth speed“Hi”. It becomes fast in order of the first speed “Lo”, the second speed“Mid1”, the third speed “Mid2” and the fourth speed “Hi”, and the firstspeed “Lo”, the second speed “Mid1”, the third speed “Mid2” and thefourth speed “Hi” correspond to the maximum capacity, the first middlecapacity, the second middle capacity and the minimum capacityrespectively. When the speed is changed in order of the first speed“Lo”, the second speed “Mid1”, the third speed “Mid2” and the fourthspeed “Hi”, the rotation speed of the left caterpillar 100 correspondingto the hydraulic motor 9 changes in order of the minimum rotation speed,the first middle speed, the second middle speed and the maximum rotationspeed.

A high-pressure selecting valve 161 is a selector valve for selectingthe pressure oil of a high pressure between a pressure of the pressureoil in the pipe 25 and a pressure of the pressure oil in the pipe 26.When the pressure oil in the pipe 25 and the pressure oil in the pipe 26have the same pressure, both of them are selected.

The pressure oil selected by the high-pressure selecting valve 161 isdischarged to a pipe 162.

The pipe 162 is connected to a 2-position selector valve 163 and a3-position selector valve 164.

A switch 165 is a switch which is operated from outside to select thespeed “Lo”, “Mid1”, “Mid2” or “Hi” of the HST vehicle and to output asignal for switching the valve positions of the 2-position selectorvalve 163 and 3-position selector valve 164 to the 2-position selectorvalve 163 and the 3-position selector valve 164. According to theoperation of the switch 165, a pilot pressure oil is supplied to the2-position selector valve 163 and the 3-position selector valve 164. Itmay be configured to switch between the 2-position selector valve 163and the 3-position selector valve 164 by sending an electric signal tothe 2-position selector valve 163 and the 3-position selector valve 164according to the operation of the switch 165.

The 2-position selector valve 163 has two valve positions, namelymiddle/maximum position 163 a and minimum position 163 b. The 2-positionselector valve 163 is supplied with a pilot pressure oil according tothe operation of the switch 165, and the valve position is switched.When the valve position of the 2-position selector valve 163 is switchedto the middle/maximum position 163 a, the pipe 162 is communicated withfirst pressure-receiving chamber 13, and when it is switched to theminimum position 163 b, the tank 24 is communicated with the firstpressure-receiving chamber 13.

The 3-position selector valve 164 has three valve positions, namelymaximum position 164 a, first middle position 164 b, and secondmiddle/minimum position 164 c. Similar to the 2-position selector valve163, the 3-position selector valve 164 is supplied with the pilotpressure oil according to the operation of the switch 165, and the valveposition is switched. When the valve position of the 3-position selectorvalve 164 is switched to the maximum position 164 a, the pipe 162 iscommunicated with the second pressure-receiving chamber 14, and the tank24, the third pressure-receiving chamber 15 and the fourthpressure-receiving chamber 68 are communicated to one another. When thevalve position of the 3-position selector valve 164 is switched to thefirst middle position 164 b, the pipe 162, the second pressure-receivingchamber 14 and the fourth pressure-receiving chamber 68 are communicatedto one another, and the tank 24 is communicated with the thirdpressure-receiving chamber 15. When the valve position of the 3-positionselector valve 164 is switched to the second middle/minimum position 164c, the pipe 162 is communicated with the third pressure-receivingchamber 15, and the tank 24, the second pressure-receiving chamber 14and the fourth pressure-receiving chamber 68 are communicated to oneanother.

Then, descriptions will be made about relations between the valvepositions of the selector valves 163, 164 and the supply of the pressureoil to the pressure-receiving chambers 13, 14, 15, 68 when the motormechanism 160 is changed its speed among the four speeds, namely thefirst speed “Lo”, the second speed “Mid1”, the third speed “Mid2” andthe fourth speed “Hi”.

FIG. 21 is a diagram showing the relations among ON/OFF (highpressure/low pressure) of the supply of the pressure oil to the first,second, third and fourth pressure-receiving chambers 13, 14, 15, 68, thecapacity of the hydraulic motor 9 and the speeds selected by the switch165. Similar to FIG. 15, “ON” indicates that the pressure oil of a highpressure is supplied to the pressure-receiving chamber, and “OFF”indicates that the pressure oil of a low pressure is supplied to thepressure-receiving chamber. The low pressure state is, for example, astate that the supply of the pressure oil is stopped. In FIG. 21, thecombination of the first columns (ON, ON, OFF, OFF) corresponds to thatof the first columns of FIG. 18. The combination of the second columns(ON, ON, OFF, ON) corresponds to the combination of the second columnsof FIG. 18. The combination of the third columns (ON, OFF, ON, OFF)corresponds to the combination of the third columns of FIG. 18. Thecombination of the fourth columns (OFF, OFF, ON, OFF) corresponds to thecombination of the fourth columns of FIG. 18.

Thus, the combinations of the supply of the pressure oil to thepressure-receiving chambers 13, 14, 15, 68 are previously determined.When any speed is selected by the switch 165, a pilot signal is outputto the 2-position selector valve 163 and the 3-position selector valve164 so that the pressure of the pressure oil supplied to thepressure-receiving chambers 13, 14, 15, 68 has a combinationcorresponding to the selected speed.

When the first speed “Lo” is selected by the switch 165, a pilotpressure for changing the speed of the hydraulic motor 9 to the firstspeed is entered the 2-position selector valve 163 and the 3-positionselector valve 164. As a result, the valve position of the 2-positionselector valve 163 is switched to the middle/maximum position 163 a, andthe valve position of the 3-position selector valve 164 is switched tothe maximum position 164 a. Therefore, the pressure oils of highpressure (ON), high pressure (ON), low pressure (OFF) and low pressure(OFF) are supplied to the first, second, third and fourthpressure-receiving chambers 13, 14, 15, 68 of the motor mechanism 160respectively. Thus, the hydraulic motor 9 has a state positioned at themaximum capacity position shown in FIG. 17(a). At this point, the centershaft 10 of the hydraulic motor 9 is changed to have an anglecorresponding to the maximum capacity position.

The hydraulic motor 9′ also operates in the same way.

Thus, the capacity positions of the left and right hydraulic motors 9,9′ are switched to the maximum capacity position, and the left and righthydraulic motors 9, 9′ are changed their speeds to the first speed “Lo”at the minimum rotation speed with maximum torque. Accordingly, the leftand right caterpillars 100, 100′ have the minimum rotation speed.

When the second speed “Mid1” is selected by the switch 165, a pilotpressure for changing the hydraulic motor 9 to the second speed isentered the 2-position selector valve 163 and the 3-position selectorvalve 164. The valve position of the 2-position selector valve 163 isswitched to the middle/maximum position 163 a, and the valve position ofthe 3-position selector valve 164 is switched to the first middleposition 164 b. Therefore, the pressure oils of high pressure (ON), highpressure (ON), low pressure (OFF) and high pressure (ON) are supplied tothe first, second, third and fourth pressure-receiving chambers 13, 14,15, 68 of the motor mechanism 160. Thus, the hydraulic motor 9 has astate positioned at the first middle capacity position shown in FIG.17(b). At this point, the center shaft 10 of the hydraulic motor 9 ischanged to have an angle corresponding to the first middle capacityposition.

The hydraulic motor 9′ also operates in the same way.

Thus, the capacity positions of the left and right hydraulic motors 9,9′ are switched to the first middle capacity position, and the left andright hydraulic motors 9, 9′ are changed their speeds to the secondspeed “Mid”. The left and right caterpillars 100, 100′ have the firstmiddle rotation speed.

When the third speed “Mid2” is selected by the switch 165, a pilotpressure for changing the hydraulic motor 9 to the third speed isentered the 2-position selector valve 163 and the 3-position selectorvalve 164. The valve position of the 2-position selector valve 163 isswitched to the middle/maximum position 163 a, and the valve position ofthe 3-position selector valve 164 is switched to the secondmiddle/minimum position 164 c. Therefore, the pressure oils of highpressure (ON), low pressure (OFF), high pressure (ON) and low pressure(OFF) are supplied to the first, second, third and fourthpressure-receiving chambers of the motor mechanism 160. Thus, thehydraulic motor 9 has a state positioned at the second middle capacityposition shown in FIG. 17(c). At this point, the center shaft 10 of thehydraulic motor 9 is changed to have an angle corresponding to thesecond middle capacity position.

The hydraulic motor 9′ also operates in the same way.

Thus, the capacity positions of the left and right hydraulic motors 9,9′ are switched to the second middle capacity position, and the left andright hydraulic motors 9, 9′ are changed their speeds to the third speed“Mid2”. Thus, the left and right caterpillars 100, 100′ have the secondmiddle rotation speed.

When the fourth speed “Hi” is selected by the switch 165, a pilotpressure for changing the speed of the hydraulic motor 9 to the fourthspeed is entered the 2-position selector valve 163 and the 3-positionselector valve 164. The valve position of the 2-position selector valve163 is switched to the minimum position 163 b, and the valve position ofthe 3-position selector valve 164 is switched to the secondmiddle/minimum position 164 c. Therefore, the pressure oils of lowpressure (OFF), low pressure (OFF), high pressure (ON) and low pressure(OFF) are supplied to the first, second, third and fourthpressure-receiving chambers of the motor mechanism 160 as shown in FIG.21. Thus, the hydraulic motor 9 has a state positioned at the minimumcapacity position shown in FIG. 17(d). At this point, the center shaft10 of the hydraulic motor 9 is changed to have an angle corresponding tothe minimum capacity position.

The hydraulic motor 9′ also operates in the same way.

Thus, the capacity positions of the left and right hydraulic motors 9,9′ are switched to the minimum capacity position, and the left and righthydraulic motors 9, 9′ are changed to the fourth speed “Hi” at themaximum rotation speed with minimum torque. Thus, the left and rightcaterpillars 100, 100′ have the maximum rotation speed.

The aforesaid embodiment was described on the assumption that thepositioning device 80 was mounted on the HST vehicle to control itsrunning. It is not limited to the positioning device 80, and thepositioning device 85 may be also mounted on the HST vehicle to controlits running.

The above embodiment was also described on the assumption that the speedis changed manually among the four speeds. But, the speed change betweenany speeds can be made automatically.

What is claimed is:
 1. A positioning device comprising: a first pistonand a second piston in a body, the first piston and the second pistonhaving a same outside diameter, the first piston having a positionrestricted by the body and a position corresponding to a position of thesecond piston as stop positions, and the second piston having a positionrestricted by the body and a position restricted by a second pistonrestricting part as stop positions; a first pressure chamber forapplying a pressure to the second piston in a direction of the firstpiston; a second pressure chamber for applying a pressure in a directionto separate the first piston and the second piston from each other; athird pressure chamber for applying a pressure to the first piston in adirection of the second piston, a first adjusting mechanism fordynamically adjusting a stop position of the first piston; and a secondadjusting mechanism for dynamically adjusting a stop position of thesecond piston, wherein: a pressure-receiving area of the second pistonis made greater than a pressure-receiving area of the first piston tothereby make the pressure of the second piston greater than the pressureof the first piston.
 2. A capacity controller for a variabledisplacement piston machine using the positioning device of claim 1,wherein the first piston is connected to a capacity control member ofthe variable displacement piston machine to control a capacity positionof the variable displacement piston machine.
 3. A positioning device forchanging a position of a subject to be positioned depending on a movedposition of a piston, comprising: the piston which moves between bothstroke end positions to change the position of the subject to bepositioned from a minimum position to a maximum position; three or morerestricting members which are positioned at three or more middlepositions of between the both stroke end positions to restrict themovement of the piston at three or more middle positions; and positioncontrol means which changes the position of the subject to be positionedamong five or more positions by the piston and the restricting members.4. A speed changing device, comprising a variable displacement hydraulicmotor which rotatably drives rotating bodies; a hydraulic pump whichsupplies pressure oil to the variable displacement hydraulic motor; andspeed switching means which changes rotation speeds of the rotatingbodies by changing a capacity position of the variable displacementhydraulic motor, wherein the speed switching means includes: a pistonwhich changes the capacity position of the variable displacementhydraulic motor from a minimum capacity position to a maximum capacityposition by moving between both stroke end positions; three or morerestricting members which restrict the movement of the piston at threeor more middle positions by being positioned at three or more middlepositions between the both stroke end positions; and position controlmeans which changes the rotation speeds of the rotating bodies amongfive or more levels by the piston and the restricting members.
 5. Aspeed changing device of rotating bodies using the hydraulic motor ofclaim 4, wherein the position control means comprises: respectivepressure-receiving chambers which apply the pressure oil to the pistonand the three or more restricting members; and pressure oil supply meanswhich previously determines combinations of high and low pressures ofthe pressure oil supplied to the respective pressure-receiving chambersfor the respective rotation speeds of the rotating bodies and suppliesthe pressure oil having the combinations of high and low pressurescorresponding to the rotation speed to be changed to thepressure-receiving chambers, respectively.
 6. A positioning devicecomprising: a first piston, a second piston and a third piston in abody, the first piston, the second piston and the third piston having asame outside diameter, the first piston having a position correspondingto a position of the second piston and a position corresponding to aposition of the third piston as stop positions, the second piston havinga position restricted by the body and a position restricted by a secondpiston restricting part as stop positions, and the third piston having aposition restricted by the body and a position restricted by a thirdpiston restricting part as stop positions; a first pressure chamber forapplying a pressure to the second piston in a direction of the firstpiston; a second pressure chamber for applying a pressure in a directionto separate the first piston and the second piston from each other; athird pressure chamber for applying a pressure in a direction toseparate the first piston and the third piston from each other; a fourthpressure chamber for applying a pressure to the third piston in thedirection of the first piston; a first adjusting mechanism for adjustinga stop position of the second piston; and a second adjusting mechanismfor adjusting a stop position of the third piston, wherein:pressure-receiving areas of the second piston and the third piston aremade greater than a pressure-receiving area of the first piston tothereby make the pressure of the second piston and the pressure of thethird piston greater than the pressure of the first piston.
 7. Apositioning device comprising: a body having a hole; a piston sliding inthe hole; and one or more of piston restriction members moving in adirection perpendicular to a sliding direction of the piston so as to befreely movable from and to the hole, wherein: both stroke ends of thepiston and positions where one or more of the piston restricting membersexpand are determined as stop positions of the piston.